I want to express the type std::chrono::steady_clock::time_point in milliseconds.

If the C++ compiler correctly supports the C++20 standard library (*), the code below will produce the desired output.
※: As of December 2022 only works with the latest Microsoft Visual C++.

#include<iostream>
# include <chrono>

int main()
{
  auto now=std::chrono::system_clock::now();
  auto now_ms=std::chrono::time_point_cast<std::chrono::milliseconds>(now);
  std::cout<<now_ms<<std::endl;
}

Demo: https://godbolt.org/z/oWdY5eK59

In the questionnaire, std::chrono::steady_clock is used, but this clock is not directly linked to a real-world clock in exchange for providing a "never retrograde watch" warranty.You should consider it as a tool to calculate the elapsed time between two points ( duration).

If std::chrono::system_clock is normally required to obtain the current time in the operating system time.