Mercur uses OpenAPI to define the API endpoints, request and response schemas. You can use http-client package which is already defined in the project to generate types and a fetch client: 
yarn codegen
The output of this process is a set of fetch functions and types. With all of that setup, calling your API from your application becomes very easy. 
// Import the generated API function
import { vendorCreateSeller } from "@mercurjs/http-client";
// Import the generated types
import {
  VendorCreateSeller,
  VendorCreateSeller201,
} from "@mercurjs/http-client/types";

// Usage example
const createSeller = async (
  body: VendorCreateSeller
): Promise<VendorCreateSeller201> => {
  const response = await vendorCreateSeller({
    body,
  });
  // ...
};