theres nothing you can do. Just because you add an anmeone doesn't mean they'll host. They may never, they can do 1000% fine for their entire life without one so they aren't waiting for one to be introduced, especially if they never had one before they may never inquire what it is.
If you want, try not feeding for a couple of days longer than you usually do, shut off all flow and when the water is still you can feed right near where the anemone is and maybe they'll brush against it. If they do that then they will for sure stay in it.
My pair which i lost one of recently took a month and a half of mine being in the tank to find out they liked it. I recently introduced a new smaller on and the one from my old pair took to him right away luckily- the next morning i came out and the new guy was in another of the anemones- he got the crash course, but he only did because he obviously followed the leader.
Good luck