Begin by going to "All Settings" in your account with the "Gear" icon in the top right corner.
Select "Social Media" on the left hand side. You can then toggle on your preferred social media and add the appropriate links.
To add links to your social media within your email, select "Design & Colors" in the top left.
Click the dropdown menu seen below.
Select "Social Media"
This will bring you to the toggle switch to enable Social Media Links.
You then have the option to apply these social links to all emails.