var connection = @"Server={myserver};
                            Initial Catalog={mydb};Persist Security Info=False;
                            User ID={myId};Password={mypassword};MultipleActiveResultSets=False;
                            Encrypt=True;TrustServerCertificate=False;Connection Timeout=30;";

services.AddDbContext<MyContext>(options => options.UseSqlServer(connection));

public class MyController : Controller
{
    public MyController(MyContext context, SignInManager<MyUser> signInManager, ILogger<AuthController> logger)
    {
        _context = context;
        _signInManager = signInManager;
        _logger = logger;
    }

    [HttpPost("myroute")]
    public async Task<IActionResult> CreateUser([FromBody] MyUser myUser)
    {
        try
        {
            var user = _context.MyUser
                .Where(y => y.UserName == myUser.UserName)
                .FirstOrDefault();

            if (user != null)
            {
                var result = await _signInManager.UserManager.CreateAsync(myUser);

                if (result.Succeeded) return Ok();
            }
        }
        catch (Exception ex)
        {

            _logger.LogError($"Create User Error: {ex}");
        }

        return BadRequest();
    }
}

public class MyContext : DbContext
{
    public MyContext(DbContextOptions<MyContext> options)
        : base(options)
    {

    }
    public DbSet<MyApi.DataAccess.Entities.MyUser> MyUser { get; set; }

    protected override void OnModelCreating(ModelBuilder builder)
    {
        builder.Entity<MyApi.DataAccess.Entities.MyUser>()
            .ToTable("dbo.MyUsers");
    }
}