onkeypress is fired only for character keys. onkeydown will fire for all keys pressed. I found some explanation of differences between all key events here
Try it with onkeydown and it worked:
<input type="text" @onkeydown="@Enter" />
In the event handler you will have to do this (notice that I check for both Enter and NumpadEnter keys):
public void Enter(KeyboardEventArgs e)
{
if (e.Code == "Enter" || e.Code == "NumpadEnter")
{
// ...
}
}