I want to use Instagram API for the direct feature and don't have more knowledge about OAuth2.0. I know Instagram provides direct API this app is using direct API but I don't know how to implement it. I seen this question but I don't know how to work with this. I want to develop iOS application but I hope any Android or Windows developer can also help for API.
Instagram does not provide an API for direct messaging. 6tag is using Instagram's private API, which is supposed to be used only by Instagram, and not by 3rd party developers.
If you want to use the Instagram private API on your app, you may be able to use the Instagram node.js Private API library or the Instagram PHP Private API library.
I have to warn you though, that I did try to use the PHP library and Instagram didn't allow sending of direct messages, no matter how much hacking I tried to make it work. But maybe this was just me...
If you are able to use node.js, it seems that Thread.configureText can be used for sending direct messages on the node.js library.
Lastly, don't forget that you will be using something that you shouldn't be allowed to in the first place.