Can you help me write a number generation algorithm?

Question

C++ / C#

I'm not sure about the difficulty.
I need help writing a number generation algorithm. there are prime numbers p and q, their length can be up to 1024 bits.
you need to choose the number g. condition gcd(g^L mod N^2-1,N)=1, where N=p*q L-lcm(p-1,q-1). You can give at least a link to information, a hint, anything.

Answer 1

1 - in terms of large numbers https://docs.microsoft.com/en-us/dotnet/api/system...
2 - in terms of algorithms - google everything about it from the name of the cryptographic method (not even a joke, just for rare topics it is difficult to quickly find a specialist who would hang out here from idleness ..)