Sorry for the confusion. The Builder extension is behind some extensions and we’re updating it to catch the others. Specifically:
– The validation module has not yet been added to the Builder. You can do that via the Custom Attributes for meta box (via the Settings tab) like this:
It’s not intuitive yet. But it should works (you can verify by seeing the exported Code). Here is the guide to work with custom attributes (written for field settings, but also works for meta box settings).
– Regarding “Why could MB Frontend not allow in the visual interface to select post_title and post_excerpt”, my first attempt to build the extension is to make it also work with existing meta box in the backend, where post fields already exist. Adding post fields to the form will duplicate them in the backend. So, they must be removed somehow.
Anyway, can you send me an admin account to check why the code doesn’t work via contact form?