You simply need to transfer 1k items into your inventory from someplace else. If you have a bunch of Junk in your workbench you can transfer it into your inventory (don't sweat the weight limit) and then back into the workbench with the one button xfer all and repeat until the counter gets to 1k.
Building a scavenging workbench early and then cleaning house and recycling everything you can in your settlements will also help you get there.