When configuring a new condition or validator in a workflow transition, users are prompted to choose a field from the drop-down menu (example screenshot below):

The drop-down menu shows a limited list of fields to select from:

What most users don't know is they can actually type in the field name and the drop-down text box will match that to a field, if that field can be used in the condition/validator:

In my example above, the "Time Spent" field isn't listed in the drop-down menu but it can be selected if the user types in the field name manually.

I suggest instead of prompting the user to "Choose field", this text may be updated to "Choose or type in field name"