Hi @re2918,
Welcome to our Freshworks developer community! 
If the third-party system doesn’t allow a localhost URL, you could run FDK with the command, fdk run --tunnel
. It will generate an HTTPS domain that can be accessed over the internet. It will look like the following.
Establishing ngrok tunnel. Please wait...
Tunnel Open. Tunnel URL: https://e1cd-xxxxxxxxxxxxxxxxxxxxxxxxxxx-3f5b.ngrok.io
Starting local testing server at http://*:10001/
Append 'dev=true' to your Freshdesk account URL to start testing
e.g. https://domain.freshdesk.com/a/tickets/1?dev=true
Quit the server with Control-C.
To simulate product, app setup, and external events, visit - http://localhost:10001/web/test
Populating test data with events found in the app
To test the installation page, visit - http://localhost:10001/custom_configs
The tunnel domain (without any port) can be used in place of the localhost domain and port for local testing of OAuth 2.0.