The button should be the forum link in which you want to create a topic. Because the [New Topic] button is located on top of each forum page. For example in our General Forums page:

https://wpforo.com/community/general-discussions/

Ok, but I want to have a permanent button (in another place than top of each page) which will be always linking to create topic in Fashion category

I'm sorry but we don't provide custom code for custom requirements. As I've already said you should create a HTML button and link it to specific forum, it requires HTML/CSS skills.