You can set up a default Email Signature as the practice's default.
You can also set up additional Email Signatures that are used instead of the practice's default. When a non-default email signature has been set up, to apply it so it is used instead of the default, it needs to be selected in the User Profile of the relevant user(s). Refer to Managing Users.
Creating an Email in Outlook
When creating a new email in Outlook, this will use the signature configuration set up in your Outlook, and not FYI.
To use the signature from FYI, you will need to first create the email in FYI (ensuring the settings above have been implemented). Use the Draft In Outlook feature to send the email to Outlook for final edits and sending to your client. Refer to Sending Emails Immediately or Saving a Draft.