How to encrypt password on login request on fetch api to spring api?

I am totally new on security topic and fetch api so maybe it is a dumb question but I do not know that where to start. I have checked some examples but I could not find a proper way to modify both frontend and backend side. I am using fetch api on frontend api but I am just sending simple request for login but both mail and password is visible on request. My fetch api code is just a simple post request:

export class User {
    userId: number;
    email: string;
    username: string;
    password: string;

    constructor() {
    }
}

registerUser(user: User) {
    return fetch("http://localhost:8080/registerUser", {
      headers: {
        'Accept': 'application/json',
        'Content-Type': 'application/json'
      },
      method: 'POST',
      body: JSON.stringify(user)
    });
  }

It is just a simple post request and all parameters are visible and I am consuming this on spring with method:

@PostMapping("/registerUser")
    public UserDto registerUser(@RequestBody UserDto userDto) throws UserAlreadyExistException {
        if (!isNull(userService.getUserByUsername(userDto))) {
            throw new UserAlreadyExistException("User with username " + userDto.getUsername() + "already exist");
        }
        User userObj;
        userObj = userService.saveUser(modelMapper.map(userDto, User.class));
        return modelMapper.map(userObj, UserDto.class);
    }

How can I modify them or where should I start to check?