How to pop up message box even when editing a cell

Hi Experts,
The following on time message is not popping up if the user is editing a cell. How can I run the codes even if the user is editing a cell (probably a formula) without loosing the cell value/formula.

Code:

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Private Sub Workbook_Open()
    Application.Run "OnTime"   
End Sub

Sub OnTime()   
    Application.OnTime Now + TimeValue("00:10:00"), "MyMacro1"
End Sub

Sub MyMacro1()
    UName = Split(Environ("UserName"), ".")
    UName = UName(0)
    MsgBox "Hi " & UName & vbNewLine & "You have 20 minutes left to complete the tests, workbook will automatically close in 20 Minutes."
    Application.OnTime Now + TimeValue("00:10:00"), "MyMacro2"
End Sub

Sub MyMacro2()
    MsgBox "Hi " & UName & vbNewLine & "You have 10 minutes left to complete the tests, workbook will automatically close in 10 Minutes."
    Application.OnTime Now + TimeValue("00:10:00"), "CloseMacro"
End Sub

Sub CloseMacro()
    ActiveWorkbook.Save
    ActiveWorkbook.Close
End Sub

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Thanks in advance!