It’s very difficult to do it in such a small window!
There are other ways than dragging the cross in the Tool>Preview.
You can also hide some of the polygons and press Transform>S.Pivot. This sets the center for symmetry and deformations to be the center of the visible polygons.
You can also use Tool>Deformation>Offset to move the model, which has the effect of changing the center.
You can also use Tool>Deformation>Unify to permanently set the center to be the center of the entire mesh.
Thanks, but I don’t understand "You can also use Tool>Deformation>Unify to permanently set the center to be the center of the entire mesh."