Hi Sixto!
Thanks for reaching out with your BoldGrid Post and Page Builder questions.
The issue here is JetForms doesn’t have an integration for Post and Page Builder. So unlike say, WeForms or WpForms there’s no block component and since they use custom fields instead of short code to pull in their form you’ll need to switch to the WordPress Block editor to modify your form.
You can access the block editor by clicking the Post and Page builder tab towards the top of your editor:
After switching to the WordPress Editor you should see the custom fields for your JetForm. I hope this helps Sixto! Please let us know if you have any other questions for us!