How will I force the user to comment in bonita
I would do it this way:
Change the Reject Button to a Checkbox
Add a hidden field on to the form which will take the rejection comment.
When the Reject Checkbox is selected the Required Field Reject Comment is opened and must then be completed.
In your process Output Operations you can then use the API to fill in the default process Comment Field (tab) as required.
PS: As this reply offers an answer your question, and if you like it, please Mark UP and/or as Resolved.
How about a modal comment do you know how to do it?