castling

how do I castle in online chess,, seems like the board wont allow..

I clicked on both the king and the rook to the left. and tried to drag the rook to the space where the king is.. but the board would not allow it,, I tried clicking on my king and moving it to the left and then trying to move the rook but the board would not allow.. how is this move (castling) executed in online chess?

PLease advise,,

Thank you

Normally by just moving the king to the appropriate square the computer should move the rook automatically. 

once there is nothing between rook and king, either side, move king two squares,
and the cumputer will automatically move rook.This move can not be done
when your in check,

point and click on the king and move it to where it would go if castling. If the move is legal, the computer will let  you make it.

maybe opponent has another piece on board that is aiming between the king and rook.

you are not going to move the rook, you have to move the king.