On this website, you are able to send a link to a media file to your Kodi instance.
For this to work you need to enable the webserver, that is included in Kodi.
Now you need to setup your Kodi instance by clicking "Add a Kodi instance". You can setup as many Kodis as you wish.
If you want to send a Youtube link, the Youtube plugin must be installed in your Kodi instance.
Be aware that - if you allow cookies - the URLs to your Kodi instances are stored in a cookie in plaintext. If you include your login information in the URL, your password may be compromised.
However, with using this site, none of your entered information will leave your local network.
You can setup custom JSON data to get transmitted to Kodi. Check out the JSON-RPC_API in the Kodi wiki.
When clicking "Add your own JSON", the JSON field will be populated with the JSON data from the selected command. Be aware, that the JSON for sending Youtube URLs will not work out of the box, because it can only handle video IDs instead of whole URLs. The ID will be automatically extracted when using the predefined command "Send Youtube URL".
The URL entered in the first input field, doesn't need to be a URL. Actually it's just a variable, you can insert into your JSON data. For convenience it's called "URL", since the main purpose of this site is to send URLs.
You can find the source code repo for this website here.
As a starting point, I used this script from the Github user DomSless.
I'm not affiliated or endorsed by Kodi.
This is the first time I did anything with JS, so I'm pretty sure, there are many better ways to achieve what I did.
But it's working, so enjoy!