Because the OpenAPI for Excel add-in is basically an API wrapper, every generic function's first argument is always the endpoint to which a request is sent, including query string parameters. Subsequent arguments depend on multiple factors: the function and its method (GET, PATCH, PUT, etc.), and which endpoint the request is sent to. Endpoints returning data require fields to be specified in order to populate output arrays (see Data Model). The Formula Builder is a useful tool for exploring the correct syntax for function arguments.
Besides the basic HTML functions, some utility functions require specific arguments detailed in the Insert Function dialog (see below). The functions include documentation to guide the user: