ShipRush has a native integration with Zentail, so all you need to do is

1. Add a webstore in ShipRush for Zentail

2. In Zentail on your API Access page (Account Settings > API Access) generate an API Token for that Webstore

 -- I have my API Key!

3. Enter the API Token in Zentail on your API Access page (Account Settings > API Access)

Note: if you have an existing API token, you can use Zentail's chat feature to request an additional API token.