Perhaps this is already known or by design but the interface does not offer the option of using a string variable as a parameter. This is reserved for numeric or digital parameter types.
Just FYI, I can work around it.
It actually does. Someone asked this question yesterday, and here is how you do it:
tl;dr - you can just type %variable-name%
directly in your text (no drop-down needed since, unlike the other inputs, this one already allows you to type in arbitrary text)
Thank you. I had seen that %variable-name% listed as an option on some other screens but because it was not listed here I did not try it. My mistake, but now I know better.