MapPost: Parameters are null

This is my Post endpoint

app.MapPost("/register", (UserRepo userRepo, RegisterDto RegisterDto ) =>
{
  ...
}

app.MapPost("/register", (UserRepo userRepo, RegisterDto RegisterDto ) =>
{
  ...
}

When sending a Request with this Payload

{RegisterDto: {Name: "Herbert"}}

{RegisterDto: {Name: "Herbert"}}

the Endpoint is hit, but the Name-Property in RegisterDto is still null
with [FromBody] and [AsParameters] i have the same issue

 public class RegisterDto
    {
        public string Name { get; set; }
    }

 public class RegisterDto
    {
        public string Name { get; set; }
    }

What does the JSON have to look like so that it works?