Would like to check if this is an option, I checked the SSL certificate of the client and it seems they have a private root. Based on one of my colleagues, it is possible to add the intermediate and private root of the client certificate to the OS root certificates to be recognized as legitimate internal SSL certificates of this client. The client's environment is on-premise.
Yes. Here you will find the documentation on how to install (root) certificates in a self-managed environment. https://success.outsystems.com/documentation/11/setup_and_maintain_your_outsystems_infrastructure/setting_up_outsystems/how_to_install_an_ssl_certificate_in_self_managed_environments/
In order that clients (browsers) accept a ssl certificate as trustest they also must have the root certificate in their trusted root store. External clients that do not have the trust certificate however will fail to trust a private ssl certificate.
Best
Stefan