how to get transit gateway default association route table id

Hello.

As far as I know, there is currently no way to get the ID of the Transit Gateway route table created by default in CloudFormation.

Yea I don't see there's a way to check it via CloudFormation but you can use CLI or console to check the same:

CLI Example:

> aws ec2 describe-transit-gateway-route-tables
{
    "TransitGatewayRouteTables": [
        {
            "TransitGatewayRouteTableId": "tgw-rtb-abcdehgh",
            "TransitGatewayId": "tgw-abcdehgh",
            "State": "available",
            "DefaultAssociationRouteTable": true,
            "DefaultPropagationRouteTable": true,
            "CreationTime": "2023-01-1T22:22:22+00:00",
            "Tags": []
        }
    ]
}

Reference: https://awscli.amazonaws.com/v2/documentation/api/latest/reference/ec2/describe-transit-gateway-route-tables.html