At a quick look it seems likely that this facility is only available for 'aggregated' podcasts - an iTunes page containing a number of podcasts from one provider, as with NPR or the BBC. I assume you have only the one podcast, so it's probably not available to you, though you could ask Support: go to https://itunespodcasts.zendesk.com/hc/en-us/requests/new and click the disclosure triangle at the right of the 'Choose a topic' field - choose 'Other' and proceed from there.
There is of course nothing to stop you creating your own short URL to redirect to iTunes: you should get your own domain name (it's not particularly expensive) with some webspace. Create a plain text with this content:
<HTML><HEAD><meta http-equiv="refresh" content="0;url=FULL URL OF YOUR ITUNES STORE PAGE"></HEAD><BODY></BODY></HTML>
Call the file index.html (make sure your text editor doesn't append .txt) and upload it to a directory on your webspace called, for example, Podcast. Then typing in your domain name/Podcast will take you straight to the iTunes Store web page.