UnDP and Custom Property

I am trying to create an Alert 

Percent of used connections

(UNDP_connections_used * 100) / UnDP_MaxLicConnections

This works if fact used on man of my platforms but . . . .

We have new platform which I can not get the UnDP_MaxLicConnections 
I have a custom property that has this value

SO I want use a UNDP value AND Custom Property

(UNDP_connections_used * 100) / CP_MaxLicConnections

Is this possible?

If so how when setting up the alert

If not any suggestions?

Thanks for the time