You can find your Vultr API token by within the Vultr Control Panel. Head to Vultr.com and either login, or create an account if you don't already have one.
In the top right, click your name with the drop down arrow to load up a pop up menu and select API. Under "Personal Access Token" it should say API access is currently disabled with an option to enable it. Click the button to enable API access.
API access will now be available and an API token will be visible and should look similar to
BNTMWIHJNGE3TZFH4XDAU3WOPX67EW7PW64A. Copy this and save it to your Vultr server provider within ServerAuth platform.
Below this API token, a new section should have appeared labeled "Access Control". This allows you to restrict which IP addresses are allowed to use your API token. Either click the button to "Allow All IPv4" or alternatively add the ServerAuth IP addresses to your acccount.