A Possession call under rule 9.5.1 (iii) is made without blowing the whistle.
There is no set hand signal – it is very effective to indicate with both hands forward; as if you were grabbing the ball (hands ball width from each other) – at the same time saying “Possession – GA”!
If the umpire needs to blow the whistle then a possession call is not appropriate.
If the call is given in a timely manner the player should remove their hands from the ball. If they do not, the umpire should penalise under the contact rule.