Hey there! You'll want to click "Settings" on the left, then "Identities" from the column that appears, then click the email account, and then you'll see the area where you can add the signature. Here is a screenshot showing this on the new interface:
Hey there! You'll want to click "Settings" on the left, then "Identities" from the column that appears, then click the email account, and then you'll see the area where you can add the signature. Here is a screenshot showing this on the new interface: