Hands equipment

Show custom entity hands equipment

Open your .iaentitymodel model file with Blockbench.

Select the base bone of the entity hands.

Create a new group which will be a child of the hand bone.

Rightclick on the bone and select "Bone Config", in this case left_hand_slot

Check the "Left hand pivot" option and press "Confirm".

The hand pivot bone has a new icon now, as you can see.

Follow the same steps to specify the the right hand, then you have to export the model as usual.