Find your .fit file from your FTP test - on your computer, it is in
…\Documents\Zwift\Activities
They are named by the date. Download and install Golden Cheetah, it is a free program. Import this .fit file from the File → import menu. It will calculate your CP from a fit to your workout’s power-duration curve. This is Golden Cheetah’s equivalent of FTP.
A more advanced option is to purchase a license of WKO5, it is much better supported and in some ways more advanced program, costs in the ballpark of $170, but you probably are not into deep analysis of your data yet.
If you were riding around 235-240-ish, and maintained this power all 20 mins, you FTP will be around 220 watt.