Where do I change the link on the sign up button?

On the Community Theme, I can't find the section where I change the sign up link.

I've found the section I need:
<input id="signup-submit" type="button" onclick="location.target='_blank' href='http://www.writers-site.com/register'" value="Sign Up" name="signup-submit">

But I can't find which file that is in.

I just want to change the URL to: