ClassicEditor
.create( editorElement, {
ckfinder: {
uploadUrl: \'my_server_url\'
}
} )
.then( ... )
.catch( ... );
The ckfinder.uploadUrl property configures the CKFinderUploadAdapter plugin. This plugin is responsible for communication with the CKFinder's server-side connector.
So, in other words, your server should run the CKFinder's server-side connector. This is a proprietary software, so I won't go deeper into how it works.
If you wish to learn about all ways to configure image upload, please read How to enable image upload support in CKEditor 5?.