If you want to have an automatically filled field, it´s just a few clicks to have this generated:
1. Create an Autonumber field on the listing object:
To make sure that this is working for all listing, we recommend to check "Generate Auto Number for existing records" during the field creation.
2. Create a new workflow rule which triggers if the field is not filled yet:
Depending which Propertybase field packages are installed in your ORG you need to create a field update for all broker´s listing ID fields (in this example there are two).
Do not forget to activate the rule and whenever you save now a listing where the broker´s listing ID field is not filled, it will be filled with your autonumber.