Instagram really makes it hard to connect an account. That's why connecting via your Instagram's username and password does not always work. The Storrito's browser extension will copy the browser cookie that is used by Instagram's authentication. Thereby Storrito can do story posts on your account. You can always disconnect Storrito from your Instagram account by changing your Instagram password, since this will make all Instagram authentication cookies invalid.
@Emily thanks a lot for asking this question via our support chat.