When you add a target, Probely will only scan pages under that target, i.e., it will not scan any pages from a different hostname.
This is what the setting "Add Site to Target" is for. If you have an API residing on a different hostname, you can add it there. This way, our scanner will scan the API as well.
Technically speaking, what this option does, is:
Any XMLHttpRequest performed to a host added under this setting, is followed and scanned.
In order to add an extra host, just access your target's Settings page and under the Scanner tab locate the "Add Site to Target" module:
Once you've done so, simply paste your API's host and click OK.