The dynamoDB provides low-level https API, so that we can access it from CA APIM gateway.
But it requires AWS 4 signing process.
The AWS4 Signer assertion can sign the dynamoDB requests.

NOTE: As AWS4 Signer is a tactical assertion, you will need to open a support ticket to acquire the assertion.

Release:
Component: APIGTW

Here is an example to configure AWS4 Signer assertion, and route via https assertion,

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More information can be found from,
https://communities.ca.com/message/242127246-re-lookup-in-dynamodb-from-apigw?commentID=242127246

Amazon documents,
https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Programming.LowLevelAPI.html
https://docs.aws.amazon.com/general/latest/gr/sigv4-signed-request-examples.html#sig-v4-examples-post