The way I understand it now, to create a BUR within an existing topic, I have to put the topic id into the field "Forum topic" at the bottom of the form.
It is not really intuitive to figure this out on your own, especially if that field is not documented anywhere.
I've found the wiki page, which describes how BURs work, but there is no mention of the input field "Forum topic" there.
This also seems to be inconsistent with other parts of this site, like the "Reply" feature at the top of the page on each topic.
That shows a numeric field called "Topic ID" with the current topic id already filled out.
With such a name, it is much easier to figure out that I need to put the topic Id into that field.
I suggest adding a new button called "Request BUR in topic" similar to the "Reply" button at the top. (Or slightly changing how the button "Request BUR" works)
This button would then forward the user to the BUR creation page, but with the field "Forum topic" (Ideally renamed to Topic ID and changed to a numeric field) already filled out with the topic Id.
Updated