Create child process with code?

Question

C++ / C#

Hello.
You need to create a child process with its own entry point.
How to do this?
For example, there is a foreign process, let's say calc.exe
And I create a child process in it with the name "calc2.exe"
And the code is executed in it:
MessageBox(NULL, "123", "123", MB_OK);
Can this be done?