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For VBA the event fired when loading a userform is Initialize

Private Sub UserForm_Initialize()

    Dim Row1 As Integer
    Dim Row2 As Integer
    
    Row1 = Sheets("Database").Range("F5")
    '(contents of Sheets("Database").Range("F5") is 1)
    Row2 = Sheets("Database").Range("F6")
    '(contents of Sheets("Database").Range("F6") is 10)
    
    With ThisWorkbook.ActiveSheet
        ComboBox1.RowSource = "'" & .Name & "'!" & "A" & Row1 & ":A" & Row2
    End With
    
End Sub
The combobox has a rowsource property to take a range reference.