The Edge SWG in Reverse Proxy has the option to forward client certificate as an HTTP header. You need to understand what the header looks like to know what you can use in that header on the receiving end.
The HTTP header uses keypairs
Client-Cert: sno=<serial_number>&subject=<subject_information>&validfrom=<validfrom_date_and_time>&validto=<validto_date_and_time>&issuer=<issuer_data>&policy_oids=<oids>
Each value will be URL encoded. If the value is not present, the keypair will not be presented.
HTTPS Reverse Proxy Admin Guide:
https://techdocs.broadcom.com/us/en/symantec-security-software/web-and-network-security/edge-swg/7-3/getting-started/page-help-configuration/page-help-services/page-help-proxy-services/page-help-proxy-services-service/HTTPS-reverse-proxy.html