If it is in the furnace room it is a supplyThat's fine. Just make sure its the return register and not the supply.
It's a supply vent. I have a T vent I could add and would probably buy solid venting. Been using professional sealing tapefind a return register and use that. i don't think duct taping flexduct to a register would work since moisture can loosen the tape's adhesive and potential leaks. use hvac tape if you decide to do that or you can order a duct reducer for under $30 and have your flexduct connect to that. just take off the register and install the reducer then connect your flexduct.