@rohit2926 please see my answer in the below post:

https://community.servicenow.com/community?id=community_question&sys_id=cf74162ddbc4bf48d82ffb2439961954

In short you cannot edit or view the code for the OOB source_request UI page. You can however create new functionality and a new page which utilizes the OOB functionality as per your requirements. Is is definitely more work as you mostly have to 're-invent' the wheel so to speak but it is possible.