++You need to configure decryption to be able to read http header insertion by firewall.
++Then follow below steps Ref Link : https://help.webex.com/en-us/article/m0jby2/Configure-a-list-of-allowed-domains-to-access-Webex-while-on-your-corporate-network
++Add the HTTP header CiscoSpark-Allowed-Domains: and include a comma separated list of allowed domains. You must include the destination domains: identity.webex.com, identity-eu.webex.com, idbroker.webex.com, idbroker-secondary.webex.com, idbroker-b-us.webex.com, idbroker-eu.webex.com, atlas-a.wbx2.com and your proxy server includes the custom header for requests sent to these destination domains.
For example, to allow users from the example.com domain, add:
CiscoSpark-Allowed-Domains:example.com
for domain(s): identity.webex.com, identity-eu.webex.com, idbroker.webex.com, idbroker-secondary.webex.com, idbroker-b-us.webex.com, idbroker-eu.webex.com, atlas-a.wbx2.com
++Create a rule for Webex and add webex in applications and applicable security policy which we created as ttp header insertion
Attached few snap
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