Paul,
This worked marvelously and I understand it. However, in testing, I ran into an issue where some older documents, that were created from earlier versions of the template do not recognize the "External_ID" as a bookmark. The issue went away simply by opening the form field properties box and visually checked that the correct bookmark name was present (it was) and closing without changing anything. I'm concerned that there could be more documents with that same issue. Any ideas as to how to mitigate such an issue if it does arise again?

This is the code that I put into place. I added a check for un-named form fields because they are managed and referenced elsewhere in the code (this is a snippet of the whole code). Since I am adding a new form field the index number for all those fields will change. Do you have any thought on what I have below?

Sub AddCheckbox()
Dim Rng As Range, FmFld As FormField


' Checks for un-named form fields and renames them if they are only referenced by the index number.
With ActiveDocument
    If .FormFields.Count = 90 Then
        If .Bookmarks.Exists("POI") = False Then
            .FormFields(42).Name = "POI"
        End If
        If .Bookmarks.Exists("State") = False Then
            .FormFields(43).Name = "State"
        End If
        If .Bookmarks.Exists("RSVPReferral") = False Then
            .FormFields(44).Name = "Referral"
        End If
    End If
    .FormFields("POI").Enabled = True
    .FormFields("Referral").Enabled = True
    If .Bookmarks.Exists("Dropdown1") = True Then
        .FormFields("Dropdown1").Name = "AddressType"
    End If
    If .Bookmarks.Exists("Dropdown4") = True Then
        .FormFields("Dropdown4").Name = "TC_Name"
    End If
    .FormFields("Record_Date").TextInput.EditType wdCurrentTimeText, , "M/dd/yyyy h:mm am/pm", False
End With


' Checks for the existence of Active checkbox and adds when necessary.
With ActiveDocument
  .Unprotect
  With .Tables(2).Cell(3, 2)
    .Range.Bold = False
    .SetWidth ColumnWidth:=InchesToPoints(2.3), RulerStyle:=wdAdjustFirstColumn
  End With
  If .Bookmarks.Exists("ExtID_Checkbox") = False Then
    Set Rng = .Bookmarks("External_ID").Range
    With Rng
      .Collapse wdCollapseEnd
      .Text = " Active: "
      .Collapse wdCollapseEnd
      Set FmFld = .FormFields.Add(.Duplicate, wdFieldFormCheckBox)
      FmFld.Name = "ExtID_Checkbox"
    End With
    With .FormFields("ExtID_Checkbox").CheckBox
        .AutoSize = False
        .Size = 8
    End With
  End If
End With


End Sub
Thanks,
Thyme2Cook