formula to link imported records previously unlinked

 If this is a brand new app which you are populating for the first time you might consider this strategy. Set your key field on the events table to be the filing code and import your data for both tables. It should all link up.

Then change the key field to the record ID of the event and the system will keep the children all attached. 

The alternative is to import your events first, and then export them again but this time include the record ID. Then using VLOOKUP formulas in Excel   To calculate the correct record ID of the event, import the children items.