Hi, I've taken another route.
When the user visits a specific page, I store the page URL in a custom field for that user, and I add there the query parameters to the URL, so that it's complete. (The page is only available to logged in users)
This works, the URL that's stored in the user profile is correct: when I copy it and paste it in the browser it shows the page correctly filled with the data from the query variables.
That customer field is in the redirect, but it leads to an 'Invalid request. Please try again.' error.
So the URL is correct, but it looks like the redirect doesn't handle it properly.
This is the command:
[mb_frontend_form id='paying-member-input-details' object_id='{{ post.ID }}' post_fields='' submit_button='Save' redirect='{{ user.tg_chat_redirect }}' ]
Pls advise.