How to emulate a keystroke in DOS programmatically (in C / C ++)?

First, I'm talking about DOS, not a Windows command line application.

I want to write a procedure that will send a keystroke for the current process. My goal is to use this procedure in ISR (interrupt handler).

In other words, this is what I want:

• DOS is loading.
• I run TSR, which registers my procedure as an ISR for some interruption (and not keyboard interruption, but now suppose that it is).
• I run another program that becomes the current process.
• The above interruption occurs.
• My procedure skips and emulates a keystroke that actually never happened.
• The currently executing process receives this keystroke, as if the emulated key were pressed.

I found several sources with different examples, but I could not get anything to work for me.

I don’t know if this makes any difference, but what I am currently using to test all of this includes FreeDOS running on a VirtualBox machine and DJGPP to compile and run the code (it uses the GNU compiler, I believe )

Any help would be greatly appreciated.