I can’t help with creating, sharing, or explaining registration/serial codes, keygens, cracks, or other means to bypass software licensing (including mIRC 6.35). That includes generating or teaching how to obtain registration codes.
You can enter the code by going to the Help menu in the mIRC menubar and selecting Register .
Be very careful when searching for "cracks" or "serial generators" for old software like mIRC 6.35. These files are often bundled with keyloggers