Refer to a particular workbook

I have the following code that is intended to save and close a particular workbook after 30 mins of inactivity. I get an error however when it is run and I think its because I have other excel workbooks open. I think i need to reference the workbook I am interested in but dont know how to do so. The workbook I want to refer to is P:\Departments Online\Property\Activity\Activity.xlsm

Sub Inactivity()
Dim x As Integer
Dim MyTxtFile

x = Application.Match(Environ("username"), ActiveWorkbook.Sheets("Users").Columns("A"), 0)                                <<<<<<Error is on this line of code
        FullName = ActiveWorkbook.Sheets("Users").Cells(x, 2)
    Set fs = CreateObject("Scripting.FileSystemObject")
    Set a = fs.CreateTextFile("P:\Departments Online\Property\Activity\Inactivity.txt", True)
    a.WriteLine (FullName & " you have been kicked out of Activity due to over 30 mins of Inactivity. Please close this message.")
    MyTxtFile = Shell("C:\WINDOWS\notepad.exe P:\Departments Online\Property\Activity\Inactivity.txt", 1)
    End Sub

sktneer

My workbook does have a sheet named Users, but my issue is (I believe) that the fact is I have many workbooks open at once, so the "active" workbook at any one time is not the one with the sheet called Users on it. Which is why i want to specifically reference the workbook instead of the active one.

ah that makes sense. Yes it would be "This" workbook that has the sheet with Users in it. Will be testing later today - thank you sktneer!