how to upload image with CKEditor 5

You can write script like this,These code are all provided by CKeditor upload documrnt,So You only need to change the request url address then you can use it directly.

@section scripts{
   //I download the CKEditor page in wwwroot,So i reference it in here.
   <script src="~/ckeditor5/ckeditor.js"></script>
    
    <script>

        //These code are all copyed from Ckeditor upload document,So you can just copy them in your project.
        class MyUploadAdapter {
            constructor( loader ) {
                // The file loader instance to use during the upload. It sounds scary but do not
                // worry — the loader will be passed into the adapter later on in this guide.
                this.loader = loader;
            }

            upload() {
            return this.loader.file
                .then( file => new Promise( ( resolve, reject ) => {
                    this._initRequest();
                    this._initListeners( resolve, reject, file );
                    this._sendRequest( file );
                } ) );
            }

            // Aborts the upload process.
            abort() {
                if ( this.xhr ) {
                    this.xhr.abort();
                }
            }

            // Initializes the XMLHttpRequest object using the URL passed to the constructor.
            _initRequest() {
                const xhr = this.xhr = new XMLHttpRequest();

                // Note that your request may look different. It is up to you and your editor
                // integration to choose the right communication channel. This example uses
                // a POST request with JSON as a data structure but your configuration
                // could be different.

                xhr.open( 'POST', '@Url.Action("Your Action","You Controller")', true );
                xhr.responseType = 'json';
            }
            // Initializes XMLHttpRequest listeners.
    _initListeners( resolve, reject, file ) {
        const xhr = this.xhr;
        const loader = this.loader;
        const genericErrorText = `Couldn't upload file: ${ file.name }.`;

        xhr.addEventListener( 'error', () => reject( genericErrorText ) );
        xhr.addEventListener( 'abort', () => reject() );
        xhr.addEventListener( 'load', () => {
            const response = xhr.response;

            // This example assumes the XHR server's "response" object will come with
            // an "error" which has its own "message" that can be passed to reject()
            // in the upload promise.
            //
            // Your integration may handle upload errors in a different way so make sure
            // it is done properly. The reject() function must be called when the upload fails.
            if ( !response || response.error ) {
                return reject( response && response.error ? response.error.message : genericErrorText );
            }

            // If the upload is successful, resolve the upload promise with an object containing
            // at least the "default" URL, pointing to the image on the server.
            // This URL will be used to display the image in the content. Learn more in the
            // UploadAdapter#upload documentation.
            resolve( {
                default: response.url
            } );
        } );

        // Upload progress when it is supported. The file loader has the #uploadTotal and #uploaded
        // properties which are used e.g. to display the upload progress bar in the editor
        // user interface.
        if ( xhr.upload ) {
            xhr.upload.addEventListener( 'progress', evt => {
                if ( evt.lengthComputable ) {
                    loader.uploadTotal = evt.total;
                    loader.uploaded = evt.loaded;
                }
            } );
        }
    }
    // Prepares the data and sends the request.
    _sendRequest( file ) {
        // Prepare the form data.
        const data = new FormData();

        data.append( 'upload', file );

        // Important note: This is the right place to implement security mechanisms
        // like authentication and CSRF protection. For instance, you can use
        // XMLHttpRequest.setRequestHeader() to set the request headers containing
        // the CSRF token generated earlier by your application.

        // Send the request.
        this.xhr.send( data );
    }
        }

        function MyCustomUploadAdapterPlugin( editor ) {
            editor.plugins.get( 'FileRepository' ).createUploadAdapter = ( loader ) => {
                // Configure the URL to the upload script in your back-end here!
                return new MyUploadAdapter( loader );
            };
        }


        ClassicEditor
            .create(document.querySelector('#editor'),{
                extraPlugins: [ MyCustomUploadAdapterPlugin ],
            })
            .then(editor=>{
                window.editor = editor;
            })
            .catch(error => { console.error(error); }); 
    </script>
}