Default values for new record created from lookup window

Hi Community!

I have the following requirement:

We need to link a custom entity called reclamations to the activity. This has to be done m:n. So I have created a new link entity called ReclamationInActivity and added it as a detail to the ActivityPageV2.js.

When adding such a ReclamationInActivity record from the activity a new page opens and the user can open the reclamation lookup window there to select or create a reclamation.

When they choose to create a new reclamation, the account and contact have to be prefilled on the "new reclamation" page.

I cannot use business rules, because there is no link from the reclamation to ReclamationInActivity.

Also, using a business process is not an option, because the user would have to save the record first and we need to prefill the values.

Is there a way to achieve this by javascript somehow?

Thanks,

Robert