01. Create a new mail flow rule
Go to Exchange admin center and create a new mail flow rule. Enter name, conditions and the append disclaimer action.
02. Create an HTML signature
You need HTML code of your signature, with the right Azure AD placeholders. Go to the free signature generator
https://www.mail-signatures.com/signature-generator/ to make this as easy as possible.
03. Finish setting up the mail flow rule
Choose the fallback option, configure exceptions and make sure all is set up correctly.
04. Test how it works
Send a test email which complies with the conditions set in the mail flow rule