Correct JSON format to add a user to group using REST POST call in spHttpClient? "Input string was not in a correct format." error

I am getting an '“Input string was not in a correct format.” error after submitting the following POST call: any idea what I might be doing wrong here?

const addUserURL: string = newWeb + `/_api/web/sitegroups('${groupName}')/users`;     
const userOpts: ISPHttpClientOptions = {
      body: `{
        "@odata.type": "SP.User",
        "loginName": "${userKey}" // a.k.a.: "i:0#.f|membership|[email protected]"
    }`
    }; 

     const addUserToGroupResponse: SPHttpClientResponse = this.props.context.spHttpClient.post(
      addUserURL,
      SPHttpClient.configurations.v1,
      userOpts);

     console.log("User has been successfully added to the security group.");