How do I stop email autofill when completing forms/sign-ons on my iPhone or MacBook Air

Whenever I'm asked to submit a username that contains my email address it double adds the email address...I can type my email address up to the period for .com and nothing happens. However, when I type the period, it adds my email for a second time and then I have to go back and erase the duplicate letters/numbers. Here is what it would look like... mickey_mouse@gmail.commickey_mouse@gmail.com.

I've tried clicking on the blue hyperlink, but rather than competing my email, it does nothing. Most the time I don't see the hyperlink so both emails are entered. How can I turn this off? I see instructions for Safari, but I never use safari...I use Google.