TestController.cs

using MailerApi.Services;
using Microsoft.AspNetCore.Mvc;

namespace MailerApi.Controllers
{
    [ApiController]
    [Route("[controller]")]
    public class TestController : ControllerBase
    {
        EmailerService Mailer { get; }

        public TestController(EmailerService mailer)
        {
            Mailer = mailer;
        }

        [HttpGet]
        public IActionResult Get()
        {
            Mailer.Send("Mister Test", "test@mail.com", "Hi, it's me.", "My plain text content", "<p>My HTML content</p>");

            return Ok("ok");
        }
    }
}