Thanks for your question!
One cannot fetch a specific Offer; rather, the Offer that has to be served depends on the request parameters, profile data, geo location, etc., and from the the Offer that is served is determined dynamically based on the data. Target does not function like a CDN or CMS where one caches their content and then fetches it to apply it to specific DOM elements. So, if you want a specific offer, you should make sure to create an Activity with specific rules that would return that Offer.
Hope this was helpful!