How to Enable the comment box in form?

I cant see the comment box button in th bottom of form.i want to enable the comment box .if anyone knows please do help.thanks in Advance!i’m using version 15

in the web form try clicking the allow comment .

If you want to enable it for Doctype, then go to doctype list->open your doctype->Settings tab->Form Settings Section->disable Hide Sidebar, Menu, and Comments