To add a new button layer to your email design, you can click on any Column or Hero layer, then select the Button option from the Quick Add dropdown menu, then click the Add + button to insert the new button layer into your layout.

After styling the button to your liking, you can specify the clickable URL by clicking on your Button layer in Figma, then clicking the Settings button, and pasting your URL into the Clickable URL Link input field; this will ensure that your button opens up that URL in the browser when it's clicked