I try to copy data (from a field in a portal) to the next record in the same portal, but I can't get it to work...

Example: I have a portal with an overview of items in stock. The last record in the portal shows the latest update from the items in stock (cNewTotal). I like to have new portalrecords showing the latest 'in stock' number in a number field (InStock).

Is there anyone out there who knows how to solve this?

When you say "next record in portal," is this an existing related record or do you want it to create a new related record? The latter would require that "allow creation of related records" be checked in the relationship definition. Either way, you should be able to accomplish this using a combination of Copy/paste, Go to Field(relationship::relatedfield) and Go to Portal Row(Last/next/previous). You'll have to be more specific about the error or problem -- "can't get it to work" doesn't tell me much.

You may want to consider the alternative method of actually doing the work in the related file, i.e. do a Go To Related Records (show only related) then do your stuff in the related file. Portals can be a pain.