Create organization wide signatures and disclaimers
1.In the Microsoft 365 admin center, select Exchange.
2.Select Mail flow.
3.Select Rules.
4.Select Add a rule +, and then select Apply disclaimers.
5.On the New rule page, complete the steps.
https://learn.microsoft.com/en-us/microsoft-365/admin/setup/create-signatures-and-disclaimers?view=o365-worldwide