If you just enable
show_in_rest param for a CPT, it will use the default WordPress namespace, which is
wp/v2. In order to use a custom namespace, you have to write a custom REST Controller (which is out of the scope of the extension). Please see here for more details.
Update: In case you write a custom REST Controller, you might disable the rest support in the MB Custom Post Type extension, and add a custom code like this.