What is the code for Drag and Drop on Windows in c++?

You know sometimes you could grab a specific file and drop it onto an exe file. and the exe would read the file and do some processing and give an output how do you do that in c++? and in additon when we run that exe file we would get a window where you could drop the file onto there and it would the same work.

For example: In the above picture we could see that there is a file called "a.txt" and it is being dragged onto an exe file called "test1.exe" and it will read the file and give an output how do you do that.

or the this

where you could open the exe file and drag and drop the file onto there.

I would prefer both.

I would like the source code of the program.