RoundCube

1. Login to your Webmail at https://webmail.domain.com/
   **PS: Replace "domain.com" from the above URL to your email domain. If you email domain is example.com then https://webmail.example.com 
2. From the left panel, click on Settings > Identities. 
3. Add the desired signature at the below box.
   
4. Once done, click on Save.