Install a Standard Minecraft 1.8.3 server through the Control Panel (Servers → Select your server → Game servers → Add Game Server → Standard Minecraft 1.8.3)
As always, I would like to thank you for The Support Team, I received correct and accurate answers to my questions in a short time. It is a huge relief that my friends can also play with non official client on the server. The impression of the first few days is completely positive, I hope it will remain so in the future. I would recommend your service to others, as it is reliable and worth the price.
Thank you so much!