How can I make a POST Request in Swift with parameters using URLSession

You need to encode the JSON string as data. Then you can add it as the httpBody. Don't forget to add the token to the request.

// Encode your JSON data
let jsonString = "{ \"command\" : \"sdm.devices.commands\", \"params\" : { \"commandName\" : \"cmdValue\" } }"
guard let jsonData = jsonString.data(using: .utf8) else { return } 

// Send request
guard let url = URL(string: "https://smartdevicemanagement.googleapis.com/v1/enterprises/\(project_id)/devices") else {return}
    
var request = URLRequest(url: url)
request.httpMethod = "POST"
request.httpBody = jsonData

request.setValue("application/json", forHTTPHeaderField: "Content-Type")
request.setValue("token", forHTTPHeaderField: "Authorization") // Most likely you want to add some token here
// request.setValue("Bearer \(accessToken)", forHTTPHeaderField: "Authorization")

let task = URLSession.shared.dataTask(with: request) { (data, response, error) in
    if let error = error {
        // Handle HTTP request error
    } else if let data = data {
        // Handle HTTP request response
    } else {
        // Handle unexpected error
    }
}
task.resume()

You could try using "urlencoded" to encode your request body. Here is my test code: (note, since I do not have a paid subscription to this service I cannot fully test my code)

struct ContentView: View {
    let project_id = 123  // <-- adjust to your needs
    
    var body: some View {
        Text("testing")
            .onAppear {
                if let url = URL(string: "https://smartdevicemanagement.googleapis.com/v1/enterprises/\(project_id)/devices") {
                    doPOST(url: url)
                }
            }
    }
    
    func doPOST(url: URL) {
        var request = URLRequest(url: url)
        request.httpMethod = "POST"
        // try urlencoding
        request.setValue("application/x-www-form-urlencoded", forHTTPHeaderField: "Content-Type")
        request.setValue("token", forHTTPHeaderField: "Authorization") // <-- your api "token" here
        
        var components = URLComponents(url: url, resolvingAgainstBaseURL: false)!
        components.queryItems = [
            URLQueryItem(name: "command", value: "sdm.devices.commands"),
            URLQueryItem(name: "params", value: "{ \"commandName\" : \"cmdValue\" }")
        ]
        if let query = components.url!.query {
            print("--> query: \(query)")
            request.httpBody = Data(query.utf8)
        }
        
        let task = URLSession.shared.dataTask(with: request) { data, response, error in
            showResponse(data)  // <-- for debuging
            guard error == nil else { print("--> error: \(error)"); return }
            guard let data = data else { print("empty data"); return }
        }
        task.resume()
    }
    
    func showResponse(_ data: Data?) {
        if let data = data, let json = try? JSONSerialization.jsonObject(with: data, options: .mutableContainers), let jsonData = try? JSONSerialization.data(withJSONObject: json, options: .prettyPrinted) {
            print("\n---> response: " + String(decoding: jsonData, as: UTF8.self))
        } else {
            print("=========> error")
        }
    }
    
}

If this does not work, have a look at this doc:

https://developers.google.com/nest/device-access/reference/rest/v1/enterprises.devices/executeCommand

In particular: The URL uses gRPC Transcoding syntax. It may be relevant.

func postRequest(email: String, password: String, onsuccess: @escaping ((Loginproduct?) -> Void)) {
    let url = URL(string: "https://app.omana-app.com/api/login")!
    var request = URLRequest(url: url)
    request.httpMethod = "POST"
    request.addValue("application/json", forHTTPHeaderField: "Content-Type")
    request.addValue("sk_d3KmPtjEtdGTjChLB0ylraAb/AWgfyR9MOxFXwgEWSZsnpoIXFVDfm1Snoak0ufRe0w=", forHTTPHeaderField: "secret_key")
    request.addValue("pk_Bvk6rVC7WxbRW2O3sy4xwCEiM+n8jdT55BQv4BA0zG4IawzpbDhTVzSvj43DcvU7cM0x", forHTTPHeaderField: "publish_key")

    let params: [String: Any] = [
        "email": email,
        "password": password,
        "device_type": 2,
        "device_token": "abc",
        "role": "4"
    ]

    do {
        request.httpBody = try JSONSerialization.data(withJSONObject: params, options: [])

        let task = URLSession.shared.dataTask(with: request) { data, response, error in
            if let response = response as? HTTPURLResponse {
                let statusCode = response.statusCode
                print("status code = \(statusCode)")

                if statusCode == 200 {
                    if let data = data {
                        do {
                            let decoder = JSONDecoder()
                            let loginResponse = try decoder.decode(Loginproduct.self, from: data)

                            // Pass the decoded model to the completion handler
                            onsuccess(loginResponse)
                        } catch {
                            print("Error decoding JSON: \(error)")
                            // Pass nil in case of decoding error
                            onsuccess(nil)
                        }
                    }
                }
            } else if let error = error {
                print("Error: \(error)")
                // Pass nil in case of network error
                onsuccess(nil)
            }
        }

        task.resume()
    } catch {
        print("Error creating JSON body: \(error)")
        onsuccess(nil)
    }
}