Try the following setup:
'Place these two macros in the ThisWorkbook module
Private Sub Workbook_BeforeClose(Cancel As Boolean)
Application.OnKey "^h"
End Sub
Private Sub Workbook_Open()
Application.OnKey "^h", "LoadForm"
End Sub
'Place this macro in a standard module, module1
Sub LoadForm()
If ActiveSheet.Name = "Sheet1" Then UserForm1.Show
End Sub
This code will only work once you have saved and reopened your workbook. The code will show userform1 when CTRL+h is pressed on Sheet1.
Note: CTRL+h will do nothing on the rest of the sheets.