3rd party apps are able to upload files directly to your Stash account via the Stash API. To authenticate this integration, you'll need to create an API Key which can be used in those 3rd party apps like Formstack, Zapier, etc.
To create an API Key, go to Account > API Access and click the New API Key button. Enter a name for the API Key so you know which app it is for. We suggest creating a unique API Key for each integration.