Actions are setup and managed by you at a specific URL or within a popup to feel quick and light.
The setup URL is the same for everyone:
You'd typically include a button or link somewhere to open the setup window.
var actions = window.open('https://actions.zapier.com/<PROVIDER>/start', 'actions', 'width=650,height=700');
The setup window is optimized for 650px width, 700px height.
See it in action
custom provider action setup window.