Are you referring to box 14?

Correct, Box 14.

Yes, this is a required field for the T5008.

Here is a link that will assist you in entering in your T5008:

How do I enter T5008 slips?

Does Turbotax prompt users to fill these out? How do we know which fields are mandatory? It seems the software would have allowed me to submit the return without filling in these boxes. Thanks!

You would fill out the information as seen on your slip. Whatever boxes are filled out are the boxes you would need to generate on the software, please see the link I will add to the original answer.

Unfortunately, when combining slips because TurboTax only allows 100 T5008 slips, there is no way to include all disposition dates.