Using an Interface in Instrumentation Tests

I am using Instrumentation Tests to run some Integration Tests. I have an Interface that defines some API calls:

Service.kt

interface Service {

    @Post("user/login")
    fun login(@Body body: LoginBody): LiveData<ApiResponseWrapper<CsrfRemote>>

    ...

}

I am attempting to run the tests on a Repository which uses these calls:

AuthRepository.kt

@Singleton
class AuthRepository @Inject constructor(
    val service: Service,
    val sessionData: SharedPrefs? = null
) {

    fun login(username: String, password: String): LiveData<LoginResponse> {

        val loginBody = LoginBody(username, password)

        return Service.login(loginBody).map {

            // parses the response and returns
        }
    }
}

How would one go about instantiating an AuthRepository instance in instrumentation tests to make the login call when I need to pass in the interface Service? Specifically, I need a Service variable that I can pass when constructing AuthRepository, but I am unable to figure out how to "pass Service as a variable"?