How to include request header into pathPrefixRewrite of urlMap?

[moicci]

How to include request header into pathPrefixRewrite of urlMap?

I am building a hosting service with many FQDNs.
Cloud Stoage has HTML files in each FQDN folders like below:

- BUCKET
    - example1.com/
          - index.html
          - page2.html
    - example2.com/
          - index.html
          - page2.html
    - ...

When Browser requests https://example1.com/index.html,
Load Balancer should transfer request to backend bucket with path "/example1.com/index.html".

And YAML should be below.

defaultService: ...
hostRules:
- hosts:
  - example1.com
  pathMatcher: index
pathMatchers:
- name: index
  defaultService: ...
  pathRules:
    - paths:
    - paths:
      - /*
      routeAction:
        urlRewrite:
          pathPrefixRewrite: /example1.com/
      service: ...

But If the Load Balaner receives requests of many hosts.
I want to write YAML like below similar to nginx variable, let me know the actual syntax to handle many hosts.

defaultService: ...
hostRules:
- hosts:
  - '*'
  pathMatcher: index
pathMatchers:
- name: index
  defaultService: ...
  pathRules:
    - paths:
    - paths:
      - /*
      routeAction:
        urlRewrite:
          pathPrefixRewrite: /$host/
      service: ...

Thanks,