Where do I find the option to add additional Box 12 items (like 12b, 12c, etc.)
You don’t see box 12 on the W2 entry screen? There might be Add another row to add more box 12 lines.
Below the first box 12 entry there is a button that says "Add another box 12 item." Click that button.
I found a video. Entering box 12 information
https://ttlc.intuit.com/turbotax-support/en-us/help-article/form-w-2/entering-box-12-information-w-2/L8WpS9LaW_US_en_US