Add icon to register button

[Lilly (Lillian)]

Add an icon to the sign up button

Hello, I want to add an icon to the register button using font awesome. I looked everywhere for the html templates but I didn’t find them, so I guess they are being generated from javascript files, right? Anyone can point me where I can add an icon to the register button? For example: this: [1] to: [2] Thanks in advance