See explanation below.
Explanation
In the Microsoft 365 admin center, choose Admin centers > Exchange.
Open the EAC and go to Mail flow
Click Add +, and then click
Graphical user interface, text, application Description automatically generated

In the New rule window that appears, enter a unique name the rule.
In the Apply this rule if box, select the conditions for displaying the disclaimer. For example, select The recipient is located condition, and then select Outside the organization. If you want this rule to apply to every message that enters or leaves your organization, select [Apply to all messages].
Next to the Do the following box, select Enter text to enter the text of your disclaimer.
Click Select one, and select one of the Fallback options if the disclaimer can't be added.
Specify the audit severity level to assign the severity level that appears in the message log.
Select the mode for the rule. Select Enforce to turn on the disclaimer immediately, or select Test without Policy Tips to put a message in the message tracking log instead of adding the disclaimer.
If you have additional conditions or exceptions that you want to add, select More options at the bottom of the page, which will show additional settings. For example, to add the exception that prevents multiple disclaimers being added in an email conversation, select Add exception and then select The subject or body > Subject or body matches these text patterns, and then specify the words or phrases in your disclaimer. Or, to put your disclaimer at the top of the email message instead of the bottom, in Do the following, select Apply a disclaimer to the message > prepend a disclaimer.
11. When you're finished, click Save
Reference:
https://docs.microsoft.com/en-us/exchange/security-and-compliance/mail-flow-rules/disclaimers-signatures-foote