In the subject of your email insert the word [ENCRYPT] or [PRIVATE], followed by a space, and then your normal subject text. 
See the example below:

Microsoft handles the decryption part automatically if the email is sent to another 365 user by verifying the recipient is who they say.

If sending to an external user, a code can be requested.
Ask the recipient to follow the instructions included with the email.

Questions about encryption?  Contact the helpdesk at [email protected]