Webhook calls to ApolloServer with express

Question

I have an ApolloServer running locally and on Firebase cloud functions.

I want to send a Stripe API webhook to it.

I tried to see if I can find documentation on how to call the server directly, but all I can find are documentations on using the playground to run queries.

Suppose this is my ApolloServer:

new ApolloClient({
  uri: `localhost:4000/graphql`,
  cache: new InMemoryCache(),
});

And this is my server:

import { ApolloServer } from "apollo-server-express";
import express from "express";

import schema from "./schema";
import resolvers from "./resolvers";

import PaymentsAPI from "./datasources/payments";

const initServer = () => {
  const app = express();
  const server = new ApolloServer({
    typeDefs: schema,
    resolvers,
    dataSources: () => {
      return {
        paymentsAPI: new PaymentsAPI(),
      };
    },
    uploads: false,
    introspection: true,
    playground: true,
    context: async ({ req }) => {
      return {
        request: req,
      };
    },
  });
  server.applyMiddleware({ app, path: "/", cors: true });
  return app;
};

export default initServer;

How do I send a post request to it without using the playground... for example, using Postman.

And my second question - I believe that the webhook will be another function in my PaymentsAPI datasource / resolver / schema. Am I right? Is it a Mutation? a Query? I am assuming Mutation as I will be saving some data to the db?