Antispoofing Configuration AWS

(I'm not fully clear on what you mean in your question so my answer might not be related to your question)

By default an EC2 instance has a parameter called Source/destination checking set to enabled, which means that any packet sent from the instance network interface will be dropped unless the packet's source IP address matches the instance IP address, and similarly, any packet received by the instance network interface will be dropped unless the packet's destination IP address matches the instance IP address.

So, to allow IP spoofing you need to explicitly disable the Source/destination checking parameter.