Adding a URL to a story isn't too hard. You just have to figure out the relevant point in the story where it'll be useful for the readers. Then, insert it and maybe highlight it or use a different color or font to draw attention. And don't forget to check if the URL works before publishing the story!
It's quite simple. Just insert the URL where it makes sense within the text of your story, making sure it's relevant and not too disruptive to the flow.
In general, it is not a common practice. While there may be some rare cases where people try to appropriate or distort a fiction book for their own purposes, the vast majority of the literary community and readers oppose such actions. There are proper channels for adaptation and reinterpretation, but 'hijacking' implies an improper and unauthorized takeover which is not the norm in the world of fiction.
Another motive could be ransom. They might hijack the balloon and the people on it, and then demand a large sum of money from the families or the company that owns the balloon. They see the balloon and its passengers as a valuable asset to extort money.