Hello.
Try second way, but without <?xml version='1.0'?> . It's should work.
Code:
<doc><ok type='blank'>http://example.com</ok></doc>
But remember, new window open this way can be blocked by brower like pop-up window.
If you want to open url in current tab of browser, try this:
Code:
<doc><ok type='url'>http://example.com</ok></doc>