Protecting a Cell, from input but allow radio button commend to populate the field with the user id and timestamp

I want to protect a cell from editing but allow the cell to be populated with a radio command button that populates the cell with the user id and timestamp.  a pop up message appears when the cell has been populated (in my case approving the form).  I can certainly lock the cell but then the radio button doesn't work,  what do I need to add on the front end of marco below?

Sub Product()

  UserName = Environ("username")
    ActiveCell.FormulaR1C1 = UserName
    ActiveCell.FormulaR1C1 = " " & Now

    x = UCase(Range("$i$5").Value)
    y = UCase(Range("$i$3").Value)
    If x = y Then

    MsgBox "ERROR - APROVER ID ERROR" & vbNewLine & vbNewLine & "Product Approver ID must be different then Sales Approver ID"

    ActiveCell = vbNullString
    ActiveCell = vbNullString
    Exit Sub
    End If

    MsgBox "Approved"
End Sub
Carolynne DonohueAsked:
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You could protect your sheet with UserInterfaceOnly = True so that macros still work
for reference


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