Linking form fields to URL parameters

I have a form that provides query parameters for an external web service request.
I also have routes based on a manually entered URL that can do the same thing.
Can I easily link the two? One way I’ve considered is to grab the values from the form text fields and use them in the path helper of form_for/4. But I cannot work out how I might get to those values without some javascript.
Or am I barking up the wrong tree with this?

redirects was what I needed - available in the controller. Learning very slowly!