Lead with Out Prospect

Experts

I got a question from our business that it is possible save lead with out entering prospect number ?. for some reason, to add some flexibility to lead process, we should save lead without entering prospect number and latter prospect  number will be  entered ?. 

If I try to save lead with out entering prospect number currently it is giving error, can you help me to understand how I can overcome this issue ?. 

Thanks in advance.