1. Use Group Policy to create a firewall rule
Since Webex doesn’t have an ADMX template, you can directly create a rule for the application executable (e.g., webex.exe or webexhost.exe or whatever is relevant in your deployment)
2. Consider a Publisher rule (more robust)
If the application is signed by Cisco (which Webex typically is), you can create a firewall rule based on the signed publisher
GPO firewall rule (preferably Publisher-based) is the cleanest, supportable method.
It will stop Windows Defender Firewall from prompting after updates.