Starting from 5.4, you have the ability to have quotas by Broker (if you have more than one broker, with specific quotas).
The first thing you need to do is to create an agent, with The Broker name you want, by filling the Broker field (the same name than the one included into your DoExternalPanel links) at the bottom of the Agent properties:
Then, on Quota Monitor, once you have defined your Quota tree, go to the "Web" tab. On the ribbon > quota, the "Web Brokers" control is active:
When clicking on it, you can add the wanted "agent brokers" (multiple selection allowed):
The brokers added will appear next to the quota tree:
Please note that will not act as Quotas, but as control variables.