how to ?

Is it possible at all?

If you're talking about forcing a pose... here's a line from my Vanilla Mannequin Script Fix which plays the default idle:
debug.sendAnimationEvent(self, "idlePlayer")

The wiki suggests not using the sendAnimationEvent on normal NPCs as it can break their idle/AI. Since mannequins (theoretically) will always be in said pose, I decided this drawback wouldn't really apply... so I released it, and the mod is used by thousands uneventfully.

Alternatively, you can run a "playIdle", but you have to assign the pose you want to a declared property... which requires an ESP. (not the end of the world if your mod already uses one). Be aware that using the property names "Pose01" "Pose02" and "Pose03" will NOT work for some reason... name your properties anything but those three and it'll work as expected. (I suspect it's legacy code from Obliv-era interfering)

You should also be aware that certain poses simply will not work correctly for whatever reasons. AND... you need to have enableAI() while setting a pose, if enableAI(false) then the pose won't start... Doing the true before the animEvent, then the false right after is how it's basically done... inserting a wait delay before the false if you want the pose to cycle forward a little before freezing it.