I created a Field Group with username, password and button fields, and set the location to User.
I then use [mb_user_profile_login form_id='150358' id_username='user' id_password='password' id_submit='button' redirect='https://...../' ]. These 'user', 'password' and 'button' are the IDs of the fields.
It looks like my field group settings get ignored, as the default MB login form gets displayed.
I also tried with form_id='the field group id', but same result.