//В контроллере
[HttpGet]
public async Task<string> PaymentBTC([FromUri] string addr, [FromUri] int status)
{
    if(status == 2)
    {
        GETRequest getr = new GETRequest();
        return await getr.Send(addr);
    }
    else
    {
        return "Просроченный";
    }
}

//Класс GETRequest
public async Task<string> Send(string addres)
{
    HttpClient client = new HttpClient();
    client.DefaultRequestHeaders.TryAddWithoutValidation("Authorization", "Bearer ApiKey");
    var responseString = await client.GetStringAsync(string.Format("https://www.blockonomics.co/api/merchant_order/" + addres));
    Order order = JsonConvert.DeserializeObject<Order>(responseString);
    if(order.Status == 2)
    {
        return "Пополняем баланс";
    }
    else
    {
        return "Просроченный";
    }
}