"Please call an ambulance for me."
請為我叫救護車 is very polite in Chinese, so saying 請幫我叫救護車 would most likely be what a Chinese person would say if they're injured and they don't care about being cordial.
Like in English, if you broke your legs in half and calmly said to the man walking past, "Could you please call the ambulance on behalf of me?"
@Charlie - good. Maybe someone else can become equally picky about what your Chinese sentence means and "correct" it further. Clearly, this Duo module is not about understanding and learning Chinese, but about teaching the Chinese how they should modify their defective language to conform with English standards. :-)