Let’s say my players used Dominate Monster on a Kraken:
While the creature is Charmed, you have a Telepathic link with it as long as the two of you are on the same plane of existence. You can use this Telepathic link to issue commands to the creature while you are conscious (no action required), which it does its best to obey.
Or my Necromancy Wizard successfully commanded Stradh:
If it fails, it becomes friendly to you and obeys your commands until you use this feature again.
Can the player have the creature use its legendary actions, lair actions, and regional effects?
I’m aware that spells like True Polymorph don’t allow this. I wondered whether these do (or should!).