Maybe this cannot be done.
I have a Form where the user must provide a PO #. Once the user does that, they must upload a PDF of the PO (not relevant to my story), then they click a radio button to Approve (at which time some text shows up). Within that text, I have 3 variables: PO #, CO #, and Vendor. The CO# and Vendor show up on the form because data was populated prior to form upload:
When using the poNo in the text didn't work, I created poNoForMessage and have it calculating =poNo. Even though it shows on the page, it's not set that it can be used in my message.
BTW, the text shows up through a field rule: