You need to TLS intercept the traffic, downgrade it from HTTP/2 and then insert a header. There are instructions at https://docs.paloaltonetworks.com/pan-os/9-1/pan-os-admin/app-id/http-header-insertion/http-header-i...
Indeed, historically this is accomplished through HTTP header insertion described above.
An alternative method would be the new SaaS Security Inline subscription, some of my customers opt for the latter as it's much easier to configure and manage.
@DavidWalters2 's answer covers it in detail. If you click the link you will see next to the header value:
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