Hello Ilia 
Welcome here!
I believe the newline operator would not work in that way, and off the top of my head, I don’t have another option using the URL the way you have.
But looking at how you have written the URL, it seems you want to set a template users can use when they click the new topic link from where you set it. What I believe you can do here is use the topic template feature and set it for your preferred category where you want the new topic created, then you set this category in the pre-filled link.
How does this sound?