Kotlin no value passed for parameter when returning Unit

Question

I'm new using Kotlin as language, and I'm making a simple app in Android Studio just to learn the language. However I can't find a proper answer to this question.

I have a function (code scraped from some places and based on the default LoginActivity from Android Studio) which returns a 'Unit'

class LoginDataSource {

    private val client = OkHttpClient()

    fun login(username: String, password: String, then: ((Result<LoggedInUser>) -> Unit)) {
        try {
            val formBody = FormBody.Builder()
              .add("username", username)
              .add("password", password)
              .build()
            val request = Request.Builder()
              .url("https://api.url")
              .post(formBody)
              .build();

            client.newCall(request).enqueue(object: Callback {
                override fun onResponse(call: Call, response: Response) {

                    println("RESULT FROM HTTP CALL: ${response.code}: ${response.message}")
                    val body = response?.body?.string()
                    //Need the body here, but no code for now exists, (not the scope of this question)
                    val fakeUser = LoggedInUser(java.util.UUID.randomUUID().toString(), "Jane Doe", "1234")
                    then(Result.Success(fakeUser))

                }

                override fun onFailure(call: Call, e: IOException) {
                    Result.Error(IOException("Error logging in: ${e.message}"))
                    then(Result.Error(e))
                }
            })
        } catch (e: Throwable) {
            then(Result.Error(IOException("Error logging in", e)))
        }
    }
}

this code returns then(Result.Success(user)

Now I want to call this function, so I assume I need to do something like this:

dataSource.login(username, password) {
  if (result is Result.Success) {
     setLoggedInUser(result.data)
  }

  return result
}

But that gives the error 'No value passed for parameter 'then'. So I need to add 'then' as parameter in the function call, but I can't find how to do that. Can you give me an example for this? The way I call the function this way to avoid 'running to may tasks on the main thread' which is also hard to prevent/solve when you are new with this.