cd ckeditor5-build-classic-custom
npm install


// src/ckeditor.js
...
import SimpleUploadAdapter from '@ckeditor/ckeditor5-upload/src/adapters/simpleuploadadapter';
...

// src/ckeditor.js
...
ClassicEditor.builtinPlugins = [
    ...
    SimpleUploadAdapter
]
...

npm run build

cd app

# Check package.json to see the usage of local package ckeditor5-build-classic-custom

// package.json
...
"@ckeditor/ckeditor5-build-classic": "file:../ckeditor5-build-classic-custom",
...

npm install

// components/editor.js
...
import CKEditor from "@ckeditor/ckeditor5-react";
...

// components/editor.js
...
<CKEditor
          editor={ClassicEditor}
          config={{
            // Pass the config for SimpleUploadAdapter
            simpleUpload: {
              // The URL that the images are uploaded to.
              uploadUrl: "http://example.com",

              // Enable the XMLHttpRequest.withCredentials property.
              withCredentials: true,

              // Headers sent along with the XMLHttpRequest to the upload server.
              headers: {
                "X-CSRF-TOKEN": "CSRF-Token",
                Authorization: "Bearer <JSON Web Token>",
              },
            },
          }}
...

// pages/index.js
...
const Editor = dynamic(() => import("../components/editor"), {ssr: false})
...

npm run dev

npm run build
npm run export