Keycloak: mapping username on subject claim

I need to configure Keycloak so that it creates a JWT with claim "sub" populated with the username, instead of the default userId in sub.

It means that instead of this token:

{
    "jti": "b1384883-9b59-4788-b09f-98b40b7e3c3b",
   ...
    "sub": "fbdb4e4a-6e93-4b08-a1e7-0b7bd08520a6",
    "preferred_username": "m123456"
}

I need to receive:

 {
    "jti": "b1384883-9b59-4788-b09f-98b40b7e3c3b",
    ...
    "sub": "m123456",
    "preferred_username": "m123456"
 }

Could you please suggest how to do that?

I tried username mapper, but it adds a second "sub" claim and with the jwt is not valid.