How to simulate alt codes in C#?

Question

C++ / C#

Good evening, problem.
C# + has a linked library (InputSimulator or Global Mouse and Keyboard Library).
I need to simulate pressing ALT+Numpad codes (alt codes), but when I run

KeyboardSimulator.KeyDown(Keys.Alt);
KeyboardSimulator.KeyPress(Keys.NumPad1);
KeyboardSimulator.KeyUp(Keys.Alt);

nothing happens, only the warning sound of Windows is heard (the keyboard hijacker says that all presses pass, everything is fine).
When I run for example

KeyboardSimulator.KeyDown(Keys.Alt);
KeyboardSimulator.KeyPress(Keys.Tab);
KeyboardSimulator.KeyUp(Keys.Alt);

all is well, it works. The only problem is with the numpad.
What could be the solution?