Two main options exist:
1. Configure static default route and then redistribute it into EIGRP;
2. Configure default-network command that should point toward the network connected to gateway router (for example, it could be it's WAN subnet or just loopback host route). This command could be configured at the Enterprise edge router that have default route or full routing table. All routers in EIGRP domain will mark this newly learnt network as "candidate default". The most easily understandable explanation for me is that we should mark network as default if we know that when packet from anywhere of the inside network reach the router connected to this network it will know how to further forward this packet.
