Is it possible to use AWS RDS SQL Server as an AAG target from on premise primary?

RDS SQL Server is a managed service, so it will not be possible to add RDS Instance as an extension node to your on-premise primary instance. However, you can connect directly from your on-premise App to hosted RDS SQL Server instance in AWS. Alternatively, if you need RDS as a DR node for your on-premises primary, you can use an option like DMS (Database Migration Service) to set up on-going replication to RDS.

Ref: https://aws.amazon.com/blogs/database/cross-region-disaster-recovery-of-amazon-rds-for-sql-server/

While RDS SQL Server supports AlwaysOn Availability Group in AWS, this is not supported today with on-premises. Depending on your use case, you can deploy your SQL Server on EC2 in AWS cloud and with Distributed availability group. One availability group can be on premises, and the other availability group can be on the Amazon Web Services (AWS) Cloud on Amazon Elastic Compute Cloud (Amazon EC2) instances in a different domain. You can reference this prescriptive guidance for Migrating SQL Server to AWS using distributed availability groups for further information. https://docs.aws.amazon.com/prescriptive-guidance/latest/patterns/migrate-sql-server-to-aws-using-distributed-availability-groups.html

With Recent announcement of RDS Custom it is now possible to have an on-premise Primary Replica and Secondary replica on RDS SQL Server. This may need to be adjusted once Multi-AZ is supported for RDS Custom for SQL Server