How do you force a mock orange to bloom?

You cannot force it to bloom.

Most regular cause of no flower is pruning at the wrong time. Philadelphus (Mock Orange) flower on wood made the previous season so pruning should be done after flowering.If it has never flowered and has not been pruned try a feed of high potash fertiliser. If this does not work for next year dig it out, throw it away and by a good one, the one you have may be blind.