if “The Thingmagic app” is the mercuryAPi, there is the example writetag.c. if you use the Sparkfun library there is example3 for the EPC. Or are you using the URA (universal Reader assistant ) ? I have just tried that (HEX 45313233) and that worked. On the URA make sure to set the read and write power to a low level ( 5dbM) and put the tag on top off of the on-board antenna.