In InStream you can easily add your email signature.
Step 1: Click on the avatar in the top right corner
Click on the avatar in the top right corner. Next, chose settings from the list.
Step 2: Open “My Account Settings” tab
Open “My Account Settings” tab and scroll it down. Next, add your new email signature and click on the “Save Email Signature” button.
Read also how to integrate your account with your mailbox.