If you have any other blog or some website where there is a ton of content, use that site to apply for Adsense. Once you get approved, you can add up to 500 sites in that account (please tell me the correct number if you find any official evidence) and serve ads on those websites.
If you don't have any other website, just add a /blog to your web application, write two to three detailed articles and then try again. They'll get you approved.
But Adsense being a CPC (and CPM) network, you won't earn much.
You can use affiliate programs of products related to your niche to make just a lot more money than Adsense.