OOC:
Thorem were surprised on round 1 and user it's action trying to escape ( you rolled STR check to escape). You must now wait for round 3.
As for mechanics I believe you want to du Ready action
[ +- ] Ready action rules
Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on Your Turn, which lets you act using your Reaction before the start of your next turn.
First, you decide what perceivable circumstance will trigger your Reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. Examples include "If the Cultist steps on the trapdoor, I’ll pull the lever that opens it," and "If the Goblin steps next to me, I move away."
You still need to describe action that will be triggered and "grab" is not an action I know of.
Once again I repeat - I am too bag DM to make decisions about when to allow your improvised actions and will simply not allow them. If monster description says
A creature
within 5 feet of the cube can take an action to pull a creature or object out of the cube
That means I will only allow you to do what is written there
Please do take a look at
this and do things according to the creature description