Possible to check existens of website ?

I would like to have a url field on the CB registration form. Is it in any way possible (without being blocked by browser security) to check if the url actually exists, before letting the registration go through. I tried to add javascript to the field, but the check of the url is being block by the browser.

JavaScript should work fine as long as you're making a simple HEAD request. GET/POST will probably get blocked by CORS. Another option is CB Code Field and its Code Validation. You can perform the HTTP request from PHP and validate that it returns a 200 response code. Code Validation supports ajax (client side) validation.