Using a JavaScript extension scoped to a custom container is the same as using a JS extension scoped to a tag. The "b." notation is still available although the custom container also uses a "u.data" object to store data set via mappings.
Be sure to read the comments section at the bottom of the template to ensure you are following the requirements. Also, do not put the script tags around the code as the template is written in JS so the code does not need to be declared with script tags.
If the tag requirements are to call a script your best bet is to use the utag.libloader library instead of the code provided by the tag vendor.
Hopefully this points you in the direction needed. If not then contact your AM/DM and they will get a set of eyes on it.