Hi,
I have not tried it myself, I believe the following C++ code should call the im: protocol handler:
std::wstring url = L“
im:user@domain.com”;
ShellExecute(NULL, L"open", url.c_str(), NULL, NULL, SW_SHOW);
The documentation of the ShellExecute function can be found here:
http://msdn.microsoft.com/en-us/library/windows/desktop/bb762153(v=vs.85).aspx
Thomas