I use the following code to set up a help context sensitive ID for one of my
UDFs (so that the user can click on 'Help on this function' in the Function
Wizard and get relevant help from the compiled help file I've designed; the
UDF sits in an add-in):

Application.MacroOptions macro:="myFunction", _
HelpContextID:=100, _
HelpFile:=ThisWorkbook.Path & "\myHelpFile.chm"

I thought Application.MacroOptions code only needs to be run once so my
current method of putting this code into the Workbook_AddinInstall event for
the add-in should be OK. However, I'm finding these context-sensitive Help
IDs seem to get "forgotten" by Excel from time to time, which means this
event isn't the answer (because the user tends to only install the add-in
once).

My next choice is to put it into the Auto_Open event for the add-in so it
runs every time but this isn't allowed by Excel and I get the error message:
"Cannot edit a macro on a hidden workbook." (I'm guessing because the event
is being triggered fractionally before the add-in is fully open).

So what do I do? Is there an event on an add-in I could use that would
work? Or does anyone know something else about Help context IDs that would
help me? I find it strange that other uses of Application.MacroOptions seem
to be remembered fine (like descriptions and categories of function) and only
have to be run once.