login-user.kt

val retrofit = Retrofit.Builder()
        .baseUrl("https://igorwojda.com/app")
        .build()

val userService = retrofit.create(UserService::class.java)
val accountRepository = AccountRepository(context)

// Wee need this because userService.login function is suspended
CoroutineScope(Dispatchers.IO).launch {
    val response = userService.login("email", "password")

    if(response != null) {
        accountRepository.addAccount(response.accessToken, response.refreshToken)
    } else {
        // show error
    }
}

// Retrofit service definition
interface UserService {
    @POST("auth/login")
    suspend fun authorizeAsync(
        @Query("email") email: String,
        @Query("password") password: String
    ): LoginResponse?
}

// Data model returned by login endpoint 
// (JSON is converted by moshi converter into Kotlin data class instance)
data class LoginResponse(
    val accessToken: String,
    val refreshToken: String
)