you can do this in pretty much any language. have you tried anything and need help or you're just asking us to do your homework for you?

FILE *f = popen("curl 'https://www.random.org/passwords/?num=1&len=12&format=plain&rnd=new'", "r");
char s[14];
fread(s, 1, sizeof(s), f);
s[13] = 0;

This is not doable in c++ unfortunately. Its an old case that’s only doable in f# or turbo pascal because of string handling and cryptography export rules (and possibly copy right)

There are various ways to solve the first part and they're pretty generic, the only "C++-specific" part is whether you'll want to build the string step by step (adding characters) or build in one go then modify (build a prearranged string, for example of a specific size, then modify as needed).

To solve the second part we (you) also need to know the encryption algorithm. "Encrypt" is quite a generic word.

You could take inspiration from Theodore Tso's pwgen: https://github.com/tytso/pwgen

It's a Unix utility in C commonly used on Linux and FreeBSD to make truly random passwords. It's the first thing I thought of when reading this.