Add a signature to email templates

1. Log in to Salesforce.
2. Click App Launcher (a grid of nine dots icon) on the left side of the navigation bar at the top of your Salesforce screen, then

1. Search for Email Templates, or
2. Click View All | Email Templates

3. Select an email template.
4. Click Edit.
5. Click Insert merge field symbol "{  }" which is located on the bottom-right side of the email composer.
6. Click Sender.
7. Select Email Signature.
8. Click Insert.

Else, simply add this merge field on the email template {{{Sender.Signature}}}.

Note: {{{Sender.Signature}}} merge field currently only supports Plain Text Format signature and does not support the signature that is in HTML format.

See Also:
 Add email signature in Salesforce  
 Email Template Signature shows HTML tags on Merge Fields