To add a signature to your messages using the web version of Yahoo Mail:
1 – First of all you login your Yahoo Account
2 – Select the Gear in the upper-right corner of thr]e corner of the window then click on Setting
3 – Click on Setting and scroll down or click on ‘More Setting’
4 – Go To Left side on the screen, and click on ‘Writing Email’ option.
5 – Click the Toggle button to enable or disable a signature for your email address.
6 – Enter or edit your signature in the text box it’s means set Password.
NOTE – Yahoo Basic Mail doesn’t support text formatting options for emails or signatures. Your email signature will be displayed in plain text instead.