GLAD YOU ASKED!
First of all, there are two ways to set up a Mitel headset button. If you do NOT see the blue button in Step 1 on your phone, you may need to use the instructions on "Mitel Headset Feature Code".
1) Hit the blue button on the Mitel phone. There is only 1 blue button
2) Tap the button next to where it says SETTINGS on the telephone screen
3) Tap the button next to where it says PROGRAMMABLE KEYS on the telephone screen
4) Select an available key that you’d like to turn into a headset button. This will be one of the keys on the left side next to the handset cradle
5) A new menu will pop up. If you don’t see HEADSET listed, click the right arrow (located just below the telephone screen) to move to page 2. Select the button next to HEADSET.
6) Now select the button next to SAVE
A new button should now appear on your screen that says “Headset” that you can use for your headset button. This should enable you to use headsets through the telephone’s headset jack.
If this does NOT work, then please try this article!