[xdude]

Well I think you don't it unless you have a published website. You can put those links in forum signature. After all if there is no website you can't put a back-link. But There is another way. If you do have a blog then you can make a review about the host4vps and the sponsor with black-links to them. You can do this in a your own blog or even as a social media post. I have done that several times. Also you can post about the sponsor it relevant threads here. For example I noticed by vps sponsor has a Black Friday offer so I posted it in Black Friday offers thread.