If you are looking to add a disclaimer at the beginning or end of the message you would need to setup a mail transport rule

1. You would need to log into the Exchange Admin Center(Exchange admin center).
2. Navigate Mail Flow > Rules on the left side.
3. Click Add a Rule
4. Fill in the fields:
     a. Name: Disclaimer
     b. Apply this rule if
     c. The recipient
     d. is external/internal
     e. Sender Location: Outside the Organization
     f. Do the Following
     g. Apply a disclaimer to the message
     h. append a disclaimer
     i. Append Code
   

<table width="100%">
 <tr>
 <td bgcolor= "green" align="center"><font color="white">This email is from a US Govt Agency.</font></td>
 </tr>
</table>

    j. Fall back to action 'Ignore', 'Wrap', or 'Reject' (this is your organization call)

5. For the rule set, we want it enforced to apply and we need to set the severity to something. I set the severity to low, and everything else I left alone.

6. Click the name of the rule under the Rule column.
7. Enable the rule

8. Test it.