I have an IPA signed for ad-hoc distribution. I can install it fine with Flash Builder. I\'d like to install a release version via iTunes 12, though. Dragging it onto:
Do not use any service similar to https://www.diawi.com/ as it can potentially have huge security implications. Using this kind of process and with some clever coding skills, a third party can inject extra stuff in you application. And they are basically charging you for something that you can do yourself.
In iTunes 12.7.x, it is still possible to install an ipa directly on a device with a simple drag-n-drop. Look at @ganesh ubale' solution here or the other answers at https://stackoverflow.com/a/46520816/609862 or https://stackoverflow.com/a/46229114/609862.
The Apple developer web site also have detailed information about how to configure a web site for installing an IPA wirelessly (by simply sharing the download link).