Custom workflow reject reason field

Hi Experts,

I am trying to achieve the following with the expenses workflow:
When the approver clicks on ‘Reject’, hidden field ‘Rejected Reason’ gets enabled and also set as mandatory on the same expense record, also the cursor is set to this field for the approver.

Once the rejection reason is entered and the record is saved, the transaction should get rejected. I do not want to use suitelet or create a different page for entering rejection reason.

Please advise on how I can achieve this.

Thanks in advance