You need :
1. A scripted freebie giver asset for your scene :
Get this sign or make your own : https://store.sansar.com/listings/20c735a9-c1f5-4598-9577-c7088fe5c5ad/freebie-sign
It has the store listing script and the interaction script in it.
In the store listing script properties, you need to replace the existing product id with either the product id for your own freebie listing, or its url which you can get from the edit listing page :
In the interaction script, on>off is changed to : show
2. A hidden listing for your freebie
The listing doesn't have to be hidden, but I think it is more fun if the freebie can't just be accessed from the store.
To make a listing hidden : at the bottom of the page you need to check this box :
Et voilà, simply place the sign in your scene and happy explorers will be able to grab your item.
Thanks to Binah for making this feature available to everyone, and Thanks to Kelly for helping me set up the sign (I had never used a script).