Microsoft 365 client-side signatures are enabled when either the client-side or both option is selected in the Signature 365 setup wizard:
When either of these options are selected, and the Signature 365 add-in is installed, signatures are automatically inserted in newly composed emails in Outlook as they are created.
Please see the following article: How to install the Outlook add-in for information on installing the Signature 365 add-in.