Target URL will include scheme, hostname and any additional resource path excluding your API basePath.

Example 1:
If you have a resource route accessible at https://yourserver.com/api/v1/vehicles/:id and would like to use /v1/vehicles as the basePath, then:

Target URL you'll use on your API : https://yourserver.com/api (this is everything before the basePath)
Fortellis URL to access your route : https://api.fortellis.io/{your-org}-{environment}/v1/vehicles/:id
Target Server the request gets routed to : https://yourserver.com/api/v1/vehicles/:id

Example 2:

If you have resource route accessible at https://yourserver.com/v1/vehicles/:id and would like to use /v1/vehicles as the basePath, then:

Target URL you'll use on your API : https://yourserver.com (this is everything before the basePath)
Fortellis URL to access your route : https://api.fortellis.io/{your-org}-{environment}/v1/vehicles/:id
Target Server the request gets routed to : https://yourserver.com/v1/vehicles/:id

Notice Fortellis replaced https://api.fortellis.io/{org-namespace}-{environment} on the incoming request with your API Target URL when building the Target Server URL(that will service the request). org-namespace will be your org identifier and environment will be test/dev/empty(for prod)