You could use something like the following in your Not In List event of your combo box:
Dim db As DAO.Database
Dim newrec As DAO.Recordset
Dim strMsg As String
strMsg = "'" & NewData & "' is not an specified First Aid Action " & vbCrLf & vbCrLf
strMsg = strMsg & "Do you want to add this first aid action to Eyes group?"
strMsg = strMsg & vbCrLf & vbCrLf & "Click Yes to add or No to re-type it."
If MsgBox(strMsg, vbQuestion + vbYesNo, "Add new First Aid Action?") = vbNo Then
Response = acDataErrContinue
Else
Set db = CurrentDb
Set newrec = db.OpenRecordset("Your_Table_Name", dbOpenDynaset)
On Error Resume Next
newrec.AddNew
newrec!Description = NewData '<<<<<This is the field name in the table you want to save to.
newrec.Update
If err Then
MsgBox "An error occurred. Please try again."
Response = acDataErrContinue
Else
Response = acDataErrAdded
End If
End If
Set newrec = Nothing
Set db = Nothing
It saves the new value to the field that the combo box displays you would need to amend the message to suit your application.
Good luck. John