Sorry to step in here, but what I said about how scripts work is very relevant here.
First, the proper field to use for adding the script is page-head-code and not header. But that shouldn’t matter unless you are using <link> to add the script.
See the help file:
For even more advanced control on a per page basis, Weaver will recognize other options that you must manually define in the Edit Page Custom Fields box. These options match equivalent global settings found on the Advanced Options tab, but apply on a per page basis.
Define page-head-code, and the value contents will be added to the <HEAD> section. This will be inserted into the <HEAD> Section after the settings in the global <HEAD> Section on the Advanced Options tab. You can include specific CSS rules here, but you must wrap them in a <style>…</style> block.
Perhaps instead of focusing on getting the script loaded for a single page, it would be more productive to just get the script loaded using the more typical site-wide method of adding the script to the general Advanced Options -> <HEAD> Section box. That will load the script for every page, but that shouldn’t matter. It will then let you test the script on any page or post. Then you can move that to the per page page-head-code custom field for the pages you want to use it on.