How to set global http headers

No, not really. Since I am authenticating in a different way it doesn't work.

I tried right clicking the wsHttps and chose "Show interface viewer" --> Clicked "Service endpoints" --> Entered my UserId for "Username" and Password for "Password". I then clicked "Assign" and chose "All requests". (I am assigning to all normal requests, I don't need test requests right now). I then tried calling some WS method (which I had removed the assigned UserId and Password headers for) but it didn't work:

<s:Envelope xmlns:s="http://www.w3.org/2003/05/soap-envelope" xmlns:a="http://www.w3.org/2005/08/addressing">
   <s:Header>
      <a:Action s:mustUnderstand="1">http://www.w3.org/2005/08/addressing/soap/fault</a:Action>
   </s:Header>
   <s:Body>
      <s:Fault>
         <s:Code>
            <s:Value>s:Sender</s:Value>
         </s:Code>
         <s:Reason>
            <s:Text xml:lang="en-GB">Please provide userID and password in Header information.</s:Text>
         </s:Reason>
      </s:Fault>
   </s:Body>
</s:Envelope>

So in the web service I'm using, I have to put "UserID" in as a header parameter. In the settings you suggested, it's "Username". I don't really know what the difference are between what you suggested and by adding custom header to each WS method.