How do I create a Netsuite Workflow and edit a custom transaction line field?

I'm trying to create a workflow that sets the value of a custom transaction line field. I have the field created, but it doesn't show up when I try to set the value using a workflow. I looked for some other custom transaction lines I made but I can't see those either. Is it possible to set the value of a custom transaction line field at all, or do you need specific conditions to be met?