Disclaimer rules for M365 work exactly like signature rules mechanically; see Creating a Rule in M365, just that they use the Disclaimer tab instead of the Signature. 

This is exactly the scenario server-side stamping is built for. A disclaimer added client-side can be edited or skipped by the sender; a disclaimer applied server-side, by ExSign, cannot. If your disclaimer exists to satisfy a legal requirement, that reliability is the entire point; treat it as a hard requirement when deciding how a disclaimer rule should be deployed, not just a nice-to-have.

See Applying Legal Disclaimers for the full walkthrough.