For a Web API call via UrlFetchApp in a Google Apps Script I have to submit the HTTP client IP (via HTTP-GET) for authentification (namecheap.com API).
Is there a wa
No, it is not possible. However, Google does provide you with a range of IP addresses through which the call will be made:
64.18.0.0 - 64.18.15.255
64.233.160.0 - 64.233.191.255
66.102.0.0 - 66.102.15.255
66.249.80.0 - 66.249.95.255
72.14.192.0 - 72.14.255.255
74.125.0.0 - 74.125.255.255
173.194.0.0 - 173.194.255.255
207.126.144.0 - 207.126.159.255
209.85.128.0 - 209.85.255.255
216.239.32.0 - 216.239.63.255
Source: https://developers.google.com/apps-script/guides/jdbc#creating_other_database_connections