Tip Calculator with Javascript, HTML, SCSS

Hey guys, this is my first javascript project here. It was fun to do. The only part I found challenging was the user's ability to add a custom tip. I wasn't sure how I would trigger it since there wasn't a button linked to it in the design. I figured I'd just use the enter key to submit their custom tips. Its not the most efficient or user friendly but I couldn't think of anything else.

Let me know what approach some of you took and feel free to suggest any improvements :)

Thanks!