For purposes of debugging the back-end, I need to make calls to it from Postman.
How can I get the value of the `ca-bundle.crt` file for inclusion in the call made by Postman?
Assuming you are trying to do what you say: use Postman as a client to make requests to your API server, and not use Postman as a proxy to intercept calls from the Roku device to the API, then you shouldn't have to do anything with the Roku certificates file.
The Roku certificates bundle file contains the certificates for well-known Certificate Authorities (CAs) used to sign server certificates. It allows the Roku device to check that it is communicating with the correct server and not some man-in-the-middle hacker. Presumably your server certificate is signed by one of these CAs otherwise your Roku roUrlTransfer calls would fail.
I would imagine that Postman would have its own certificate bundle file, as do most https clients (Rokus, browsers, curl, etc), so it should have no trouble communicating with your server using https. If for some reason Postman does not recognize your server's certificate, then it does have a setting where you can turn off SSL certificate validation.