If you use the CURL HTTP client,your command to retrieve K-1 data would look like this:
curl --request GET \ --location "https://{apiGatewayAddress}/fdx/v5/tax-forms/1234567890123456" \ --header "Accept: application/json" \ --header "x-apikey: R3VgcNfo1kQTgLVR2123456778bAoyXuT9mxwlaGWw" \ --basic --user 1234567890123456:87654321