By default, SharePoint doesn’t allow us to edit the List forms in browser. See the Edit Page from Site action is greyed out! However, Its possible to edit SharePoint list form pages!
Here is the trick to edit SharePoint List Forms (like DispForm.aspx, NewForm.aspx, EditForm.aspx) in browser. Just append: &toolpaneview=2 in the URL
Now, you will get editable page, You can Add web parts!
This trick works on even SharePoint 2010 and in SharePoint 2013 also!