How to use a connected service from a custom Azure Devops extension index.ts?

The trick is to use some more functions from azure-pipelines-task-lib/task (in your case, tl):

If your custom Connected Service adds an authentication scheme of type ms.vss-endpoint.endpoint-auth-scheme-token, then the id of the token input would be apitoken, and the code you would need to add would look something like the following:

const endpoint = tl.getEndpointUrl(serviceString, true);
// The second parameter here is the name of the associated input 
// value(s) of the specific authenticationSchemes type (more on that later).
const token = tl.getEndpointAuthorizationParameter(serviceString, "apitoken", false)

How do I know this? Experimentation.

Other Authentication Types

My current experience echoes that of others' from the past: Azure DevOps does not play nicely with fully custom Connected Services. It does ship with a few that cover most bases. Depending on which one(s) you use, the value you pass for the second parameter of tl.getEndpointAuthorizationParameter changes:

ms.vss-endpoint.endpoint-auth-scheme-token

Ships with one standard input:

1. apitoken

ms.vss-endpoint.endpoint-auth-scheme-basic

Ships with two inputs:

1. username
2. password

Example Plus Suggestion

Suggestion first: to make your code clearer, rename your serviceString variable to connectedServiceId (or some variation to be clear that is represents the ID of your connected service).

import tl = require('azure-pipelines-task-lib/task');

async function run() {
    try {
        const connectedServiceId = tl.getInput('TestService', true);

        if (connectedServiceId == 'bad' || connectedServiceId == undefined) {
            tl.setResult(tl.TaskResult.Failed, 'Bad input was given');
            return;
        }

        const endpoint = tl.getEndpointUrl(connectedServiceId, true);
        const token = tl.getEndpointAuthorizationParameter(connectedServiceId, "apitoken", false)
    }
    finally {
        // Probably report some failure here, right?
    }
}

Additionally, adding the connectedServiceId == undefined check allows for safe usage of connectedServiceId in later function calls.

Examples I found helpful/created

• SonarQube's extensions
• Universal Git Tags