On the user computer, under their profile.

Click on file explorer.

Right click this PC.

Left click map network drive.

Enter the path in the folder field. Leave checkboxes as they are (reconnect at sign-in is checked and connect using different credentials is unchecked).

Click finish.

Note: to find the path to their M drive, go to AD, profile tab, and the path to the M drive is listed after "connect."