When the developer goes to your API documentation, they will see the button on it. Below is an example screen of what the button could look like.
If they click on one of the first two options, the collection will be directly imported to their Postman app and show up in the foreground.
If they click on the third option, which people who do not have Postman will do, they will be directed to the web view where they can either download a file of the collection or get Postman. Nothing will happen if someone without the app clicks on the first two options.