How do i change the anchor point of a structure to the middle

So i am making a skyblock mod, i created an item that spawn an island.

BUT the anchor point that the scructure block puts in is at a corner, and this results in depending you're facing the island rotates unesesarily.

i spawned the isles at the red point

i tried to change in the structure block but it doest have this option, so i need to get around by procedure

and i dont find ANY tutorial at all, neither explanation in forum

the procedure i am using is this

I AM IN NEED OF DIRE HELP