Content variables on Incident templates

Hello.

I would like to know if the following scenario is possible.

I created an Incident Template for Firewall requests and added a new field to the template called ip_address
I checked the content variable on the Notification Rules and the new field was there, represented by $UDF_CHAR10.
So, on the Incident Template, in the Subject field, I would like to show the following text: "Firewall request for IP: $UDF_CHAR10"

Is this possible? I'm not being successful because the emails are arriving with the text "Firewall request for IP: $UDF_CHAR10" and not with the content of the variable.

Thanks for your help.

Cheers.