Are you just looking to include a library?
Adding the script library as a 'resource' will place it into the head tag.
- On your xpage, go to the resources tab>add>javascript library
- Change the radio button to "link to javascript library from a URL"
- Paste in your URL below.
If it's a library stored within the database in the resources section, on step 2, leave the top radio utton selected and pick the library from the list below.