I’ve checked on your site and I see you created a field with ID
post_title. But as it’s a field in a meta box, it can’t be a post field.
To add post field, you need to change the shortcode to:
[mb_frontend_form id="dominantgegevens,diensten" post_fields="title"]
And the template for post title now works.
Just a note: post fields (such as post title, editor, thumbnail, etc.) are not part of meta boxes, so you can’t wrap it in tabs. They will appear above the tabs.
Here is the screenshot of the page: