Now a few ifs that might change up how I would introduce them:
If this was my only tank and I had no room for even a plastic bin of a few gallons...
If my betta was the only aquatic animal (fish, frog, etc.) in the tank...
If I never did the quarantine trio that Aquarium Co Op recommends on my betta...
If I did not have another filter, even a sponge, to put in the quarantine tank/bin
then, and pretty much only then, would I consider introducing the fish directly to the tank and treating the whole tank as a quarantine tank while administrating the medication trio as they recommend
then, I would keep my eyes on all fish due to the risk being taken
TIP: I honestly like to use the 10g aquarium co op sponge filter's sponge on my intakes for hang on back filters to keep them seeded with bacteria. When I need to quarantine I get a plastic bin, put the sponge filter together with the seeded sponge, and begin the fun. After quarantine, if all goes well, I return the sponge back to the tank after sterilizing to reseed.