This is a C++20 wrapper for the Argon2 hash function library, which is written in C, and is available as a reference implementation on GitHub. Argon2pp is a header-only project; # include "argon2hash.h" somewhere in your code will do the trick. You will of course need libargon2 and the header file argon2.h in your compiler's include path. In Debian-like systems this is found in the libargon2-dev package. Other distros will have similar packages.
The argon2hash.cpp file is a very simple test harness showing how to use the Argon2-id variant of the hash.
The license is BSD-3.