Redirect user when the worker is done
You can do it as follows:

When the user presses the button the server starts the task, and then sends a response to the client, possibly a "please wait..." type page. Along with the response the server must include a task id that references the task accessible to Javascript.
The client uses the task id to poll the server regarding task completion status through ajax. Let's say this is route /status/<taskid>. This route returns true or false as JSON. It can also return a completion percentage that you can use to render a progress bar widget.
When the server reports that the task is complete the client can issue the redirect to the completion page. If the client needs to be told what is the URL to redirect to, then the status route can include it in the JSON response.

I hope this helps!
I have the app in python, using flask and iron worker. I'm looking to implement the following scenario:

User presses the button on the site
The task is queued for the worker
Worker processes the task
Worker finishes the task, notifies my app
My app redirects the user to the new endpoint

I'm currently stuck in the middle of point 5, I have the worker successfully finishing the job and sending a POST request to the specific endpoint in my app. Now, I'd like to somehow identify which user invoked the task and redirect that user to the new endpoint in my application. How can I achieve this? I can pass all kind of data in the worker payload and then return it with the POST, the question is how do I invoke the redirect for the specific user visiting my page?