As I shared above that I got .html in page by a Plugin. The rest of the problem is to add /p/ before page name that I got also as TheDeadMedic mentioned above. I just created a page with URL www.example.com/WP/p.html and then created another desired page with URL www.example.com/WP/sample-page.html but add the second page as the child of first www.example.com/WP/p.html page. Then I open my desired page then I got the URL as I want www.example.com/WP/p/sample-page.html and I got my problem solved.
Get All Steps In Pic By wikiHow at: How to Add a Subpage in WordPress