Is it true if a rub onion juice on my face it might stimulate beard growth?

Unfortunately, this is not true, otherwise everyone would be doing it! Best option is to talk to a Hair Loss Specialist and maybe look into a hair transplant. 

no ,it is not true if a rub onion juice on the face it might stimulate beard growth.Beard area may restored with HT. You may have  a hair transplant surgery from scalp skin follicular units to beard area with FUE to have facial beard hair.If the patient needs a full face transplantation  , at least 3000-3500 grafts would better to harvest and transplant to cheek, mustache areas.

Ok - I laughed out loud when I read this.
Nope, not true.  It would be so easy if it was, though, right?  But like most things in the "hair world" believe half of what you see and none of what you hear.
Good luck!