You can try overwriting. I answered a (my own) question about how to overwrite the reciever-email with a field (dropdown) in the form.
You can find the TS for that here:
https://github.com/einpraegsam/powermail/blob/develop/Configuration/TypoScript/Main/setup.txt#L146
See my solution for the reciever here:
Send TYPO3 EXT:powermail mail to diffrent recievers depending on selection in a dropdown
After posting part of the question in slack-channel of powermail ext, Alex posted this solution:
plugin.tx_powermail.settings.setup.receiver.overwrite.subject = COA
plugin.tx_powermail.settings.setup.receiver.overwrite.subject {
10 = TEXT
10.data = GP:tx_powermail_pi1|field|markername
10.noTrimWrap = |My subject with the value ||
}