I want to buy a certain app subscription… how can I choose which one I want

I want to subscribe to Dark Room app but there’s several different options like for example; a one year subscription or one month or one week etc… but I can’t figure out how to choose which one. When I click to subscribe to the app, it automatically subscribes me to whatever it wants. How can I choose which length of time I want to subscribe for in the App Store for a specific app