The approach using Custom Auth with a JSON-RPC body that mixes static secrets and dynamic expressions is valid and supported in n8n. Based on the available documentation, Custom Auth itself isn’t a known cause of HTTP Request failures when correctly configured; issues in this area are more commonly related to invalid URLs, incorrectly resolved parameters, or credential permission/configuration problems, depending on the specific error message.
In this case, based on the screenshots, it appears likely that the HTTP Request URL is resolving to undefined, which would be consistent with the fact that credentials can’t be accessed via expressions in n8n.
Because of that, the URL should be defined directly in the HTTP Request node. Custom Auth is intended to inject headers, body, or query data, but not to provide the request URL itself.
Reference: