How could I Mock this code?

I want to start mocking some code but I am unsure how to do it. I have read a few tutorials but I cannot apply it to my code (There maybe a reason behind it). I am using NUnit and Moq. (I have removed all other methods so I can just show you one).

Api Interface:

public interface IApiRequest
{
    Task<T> ExecuteAsync<T>(RestRequest request);
}

Method I want to Mock:

public async Task<UpcomingMovies> GetUpcomingMovies(int page)
        {
            var request = new RestRequest
            {
                Resource = "movie/upcoming",
            };
            request.AddParameter("page", page.ToString());
            request.AddParameter("language", "en");

            var api = new ApiRequest();

            return await api.ExecuteAsync<UpcomingMovies>(request);
        }

I'm not sure how I can mock this.