onclick.kt

authenticateButton.setOnClickListener {
    val service = retrofit.create(TerminalService::class.java)
    val loginCall = service.login(loginEditText.text.toString(), passwordEditText.text.toString())
    loginCall.enqueue(object : Callback<ResponseBody>{
        override fun onResponse(call: Call<ResponseBody>, response: Response<ResponseBody>) {
            val token = response.body()?.string()
            if (token != null){
                Log.d("PPE_LOG", "The token is $token")
            } else {
                Log.w("PPE_LOG", "Login failed")
            }
        }

        override fun onFailure(call: Call<ResponseBody>, t: Throwable) {
            Log.e("PPE_LOG", "Login failed")
        }
    })
}

TerminalService.kt

import okhttp3.ResponseBody
import retrofit2.Call
import retrofit2.http.GET
import retrofit2.http.Query

interface TerminalService {
        @GET("login.php")
        fun login(@Query("login") login: String, @Query("mdp") password: String): Call<ResponseBody>

        @GET("reservations.php")
        fun reservations(@Query("token") token: String): Call<List<TerminalReservation>>
}