Hi, its not totaly clear which end of the strut has come of in your first post. If its the mounting point on the inner quarter panel of the car then it has a threaded nut which is welded to the inside of the panel and if its become damaged then its very tricky to fix as its done from the inside of the panel (where you cant get too)
If its the part on the inside of the tailgate, best bet would be to remove the tailgate inner trim to gain access to inside of the boot and attempt a fix with a correct nut and suitable washer on the inside and bolt up your old ball mounting back on.
If its just that either mount point has stripped its threads it probably needs re-threading with a suitable kit and a new ball mounting with a fresh thread fitting to rectify the problem.
Hth