ASP.NET Core form POST results in a HTTP 415 Unsupported Media Type response

First you need to specify in the Headers the Content-Type, for example, it can be application/json.

If you set application/json content type, then you need to send a json.

So in the body of your request you will send not form-data, not x-www-for-urlencoded but a raw json, for example {"Username": "user", "Password": "pass"}

You can adapt the example to various content types, including what you want to send.

You can use a tool like Postman or curl to play with this.

You can use [FromBody] but you need to set the Content-Type header of your request to application/json, i.e.

Content-Type: application/json

For forms, use the [FromForm] attribute instead of the [FromBody] attribute.

The below controller works with ASP.NET Core 1.1:

public class MyController : Controller
{
    [HttpPost]
    public async Task<IActionResult> Submit([FromForm] MyModel model)
    {
        //...
    }
}

Note: [FromXxx] is required if your controller is annotated with [ApiController]. For normal view controllers it can be omitted.