There are common add-on codes that are pre-programmed into our system and linked to specific services. You can add these codes to your calendar sessions, but first you need to add them as services to your account.
First, Go to Settings > Billing > Services and click "Add New Service." In the "service" box, type in a + sign:
This will bring up a list of all of the add-on codes. Select one of them, set the Rate and Duration, then save the service. Repeat this for any codes you want to add.
When you create a new session on your calendar for 90834, (or one of your other common services) you'll see the add-on options appear below this in the calendar flyout. Click on the add-on code(s) that you want to add to the session:
Note that the only options that appear will be those that you've added on your services page. Also, keep in mind that certain codes are pre-linked to other services--you can't customize which add-on codes appear for specific services. All of the codes in the example above are linked to 90834 and will appear under this code when it's selected from the calendar flyout (as long as they have first been added as an active service on your Services page).
For more info on using/adding service codes, check out this article: Using Service Codes