The problem is that you have your monitors set up wrong: Windows thinks they are in the wrong places, so it moves the mouse as you have told it.
Me? I'd set it right in Windows display settings, because it's a one-off change that is applicable to just that one system and no other.
Can you do it in C#? Possibly, but I've never tried (nor do I want to play with it). This may help: winforms - How do I enable a second monitor in C#? - Stack Overflow
] If you can enable it, you should be able to set where it is, but the link at least gives you the API references you will need.