I want populate "CustomerVar21" (cv21) with data from a map.

So I created the map and it's working OK.
The map has two columns, MAC and HostName.

In SCC->Configuration->Event Configuration I renamed cv21 to "dhcpHostName".

In "Data Source" I selected "Referenced from Map".

Map Name: <the name of my map>

Map Column: HostName

Key Configuration:
Map Key Field: MAC (Marked as "Key" in the map definition window)
Event Field: SourceMAC

What's the next step?
I generated some events that contain the "smac" field and that have a
mapping in my map.

Do I need to do something to my collectors for it to start populating
cv21 in incoming events?

I was hoping that it would work automatically, i.e. Sentinel would
populate cv21 for all events that have a smac field in my map.