Option Explicit
Public WithEvents CbEvent As ComboBox

Private Sub CbEvent_KeyDown(ByVal KeyCode As MSForms.ReturnInteger, ByVal Shift As Integer)
    MsgBox True
End Sub

Option Explicit
Dim CbEvents As New cl_CBEvents

Private Sub Workbook_Open()
    Dim sh As Worksheet
    Dim cb As MSForms.ComboBox
    Dim objTmp As OLEObject
    
    For Each sh In Worksheets
        For Each objTmp In sh.OLEObjects
            If TypeOf objTmp.Object Is MSForms.ComboBox Then
                Set cb = objTmp.Object: Exit For
            End If
        Next
    Next
    
    Set CbEvents.CbEvent = cb
End Sub

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    If Intersect(Target, Range("C:C")) Is Nothing Then
        Application.OnKey "~"
        Application.OnKey "{ENTER}"
    Else
        Application.OnKey "~", "ShowForm"
        Application.OnKey "{ENTER}", "ShowForm"
    End If
End Sub
Sub ShowForm()
    UserForm2.Show 0
End Sub