Post request work on postman but not in browser (CodeStatus : 415) - SpringBoot, thymeleaf

It's a bit a shame but I just started with front end developping.

My Problem : I have a html page with a form and the submission requestbody is not transfered to the back end.

I'm using spring boot, spring security, thymeleaf.

Here the Controller :

RegistrationController java class

package my.package;

import lombok.AllArgsConstructor;
import org.springframework.web.bind.annotation.*;

@RestController
@RequestMapping(path = "/registration")
@AllArgsConstructor
public class RegistrationController {

    private RegistrationService registrationService;

    @PostMapping
    public  String register(@RequestBody RegistrationRequest request){

        registrationService.register(request);

        return "Registration need to be confirmed";
    }

    @GetMapping(path = "confirm")
    public String confirm(@RequestParam("token") String token) {
        return registrationService.confirmToken(token);
    }

}

With PostMan the request works as a Post request with the body as json:

{
    "firstName": "firstName",
    "lastName": "lastName",
    "email": "firstName.lastName@gmail.com",
    "password": "password"
}

But when I implement the html page as following :

    <div>
                    

and fill the formular and press submit the business services dont succeed : got 415 status error

With DevTools i see the Payload nicely filled

I do not understand why the @RequestBody don't get filled with the payload data. Maybe the problem is somewhere else. I don't understand why in postman the request works but not in the browser

Thank you for all the help you can bring

Alex