Struggling with vocal melodies

So I have no problem at all creating a beat with Melodies etc. as I’ve done that 1000 times before in my 3 years of producing. However I’ve just recently dipped my toes into vocal production as well,(singing over a beat) however when I have made the beat I have no clue where to begin, I struggle hearing what notes that I can sing that are in key, and basically I never come up with a good vocal melody. Does anyone have any useful tips for someone like me? Thanks