You can provide local ASN for each tenant on the tenant VRF.
efa tenant vrf create --name <vrf-name> --tenant <tenant-name> --local-asn <local-as-for-vrf>
efa tenant vrf update --name <vrf-name> --tenant <tenant-name> --operation <local-asn-add|local-asn-delete> --local-asn <value>
Note
Ensure that the local ASN support on IPv6 AF is checked.