I'm late to the party but I had this issue, found this post without any answers, and then later found the solution! Figured I would come back and help any others that find this down the road.
To add the 'Session Key' button you need to navigate to the UI action. I did this by opening an incident that has the button in the form already (with the SOW view), right-click on the button > edit UI action. Once you're on the UI action scroll down to the workspace tab, and check the 'Workspace Form Button ' and 'Format for Configurable Workspace ' boxes. Save and reload an incident on your SOW and the button should be there!
Hope my answer helps you! First time poster, thankful to finally be able to provide an answer instead of more questions. Cheers!
