How to access Namespaced JWT Claims in AWS HTTP API Gateway Request Mapping - jwt

Question

I'm trying to set up HTTP integration in AWS API Gateway v2 (aka HTTP API). In my config, I have a native JWT authorizer and want to append one namespaced JWT access_token claims to HTTP request headers.
As long as claims as simple name such as sub or iss this is working fine with the following mapping syntax:
append:header.simple = append:$context.authorizer.claims.simple
However, some of my claims are namespace with an https://namespace/ prefix (is a requirement from Auth0 and cannot be changed). This is where mapper syntax is falling short for me.
Say my input JWT is like this:
{
"aud": "my.dev.api",
"azp": "CCCC",
"exp": "1610606942",
"https://my.ns/account_no": "100368421",
"iat": "1610598342",
"iss": "https://mytenant.auth0.com/",
"scope": "openid profile email account:admin",
"sub": "auth0|user-id"
}
How can I map namespaced claim https://my.ns/account_no?
I tried $context.authorizer.claims['https://my.ns/account_no'] with no luck. Here is the terraform setup I use:
resource "aws_apigatewayv2_integration" "root" {
api_id = aws_apigatewayv2_api.api.id
integration_type = "HTTP_PROXY"
connection_type = "INTERNET"
description = "This is our GET / integration"
integration_method = "GET"
integration_uri = "http://${aws_lb.ecs_lb.dns_name}"
passthrough_behavior = "WHEN_NO_MATCH"
request_parameters = {
"append:header.account_no" = "$context.authorizer.claims['https://my.ns/account_no']" <-- FAILING HERE
}
}
Error I'm getting in terraform and dashboard is the same:
Invalid mapping expression specified: Validation Result: warnings : [], errors : [Invalid mapping expression specified: $context.authorizer.claims["https://my.ns/account_no"]]
Thanks for your assistance.