Underscore sign doesn't display in keypad

The underscore sign in my ipad mini 2 keypad doesn't display while signing in to my email address. But the sign does show up in the keypad when I write my password. How do I access the underscore sign while writing my email address?

Here are the screen shots of the keypad:

First screen shot is the one where the '_' sign is not displayed above '123', where it should be displayed, when I try to write my email address:

The second screen shot is the where the '_' sign is displayed when I write the password.

So my question is how do I get the '_' sign to appear when I'm trying to write my email address?