"I'm really searching for the C++ code which is able to send a character to the port"

In other words, this is a PC question - so, strictly, it's off-topic here.
Try putting "mscomm32.ocx" into your favourite internet search engine.

Take a look at http://www.windmill.co.uk/

However, this comes up quite often here - so a 'Search' would be worth your while.