That entirely depends on your app, network, whether secured or open, etc… whatever app you get should have detailed directions for that, either in the app, on iTunes store or on the app maker’s webpage, etc.
Then it is up to you to do the trial and error thing until you find an app/url combo that works… there is no “one” correct way.
For example, one app i have used only needed a simple url like this http://10.10.3.23:8080/video another needed a much more complicated one… I don’t have it handy, not that it would help you anyhow 