Updating lookup

Can someone help with a bit of coding please?

I have two forms - 'Postcode Lookup' and 'Job Booking'. The 'Postcode Lookup' form has just two fields, Company and Postcode. There are approximately 100 companies in the data, but only 10 or so postcodes.

On the 'Job Booking' Form I have a lookup field which searches the Company data and if it exists, populates a Postcode field on the 'Job Booking' form along with the Company. There is also an option to add a new company if it doesn't exist.

As many of the Companies don't have matching postcodes, how can I select a company from the lookup, manually type in the postcode and have the postcode saved to the 'Postcode Lookup' form so it is there for the next time?

Also, if the Company doesn't exist we would need the company and postcode data added to the 'Postcode Lookup' on submission.

Hopefully that makes sense?

Thanks

Nick