An autonomous system number (ASN) is a unique number that's available globally to identify an autonomous system and which enables that system to exchange exterior routing information with other neighboring autonomous systems.
To enable this feature please SSH or console into the firewall and execute the following:
admin@C0EAE4E34CB4> config t
config(C0EAE4E34CB4)# routing
ARS BGP>config t
(config-routing)# bgp
ARS BGP(config)>bgp extended-asn-cap