Where are you finding this "via" option on the SWR» website? I can't see one, and of course you don't even get the "direct trains only" option until you ask for prices.
I do not use the 'phone app, but the 'via' option is fairly obvious on the website as seen on a PC/laptop....
The "via" option is there on the front page of the web site. If you do NOT enter it there, though, the result page does not offer it. You have "from" and "to" shown, you have "refine your results" which included buttons like "direct trains only", but there's no direct way at that point to refine by adding a "via" - you need to select the "edit" button.
The pages of results when you have entered a "via" do not echo that routing - there is no way from looking at the display that you can tell what the routing is, unless you can guess / work it out from the trains offered - see the first post in this thread.
An elementary web site design failure in that results pages should, as a matter of routine, include the key parameters used to generate them - otherwise you have "here is the answer but we're not telling you what the question was ...". I am, however, not here to teach SWR the principles of web interfaces. I used to to that as a job - now if they would like to pay me ....