There are 2 ways you can add Twitter to your signature:
You can add a Twitter social icon to your signature that, when clicked, will bring users right to your Twitter page.
You can also add a Social badge.
1. Go to the Add-Ons section and find the Social Buttons app. 
2. You’ll have a few different Twitter buttons to choose from, like full color or white buttons.
Choose the one you like, link your page, and save the changes to your signature.
Refresh your email and you’ll see the updated social icon or banner in your next email.