Custom Domains for Pages (saas for custom domains)

Need some guidance with Pages custom domains . I would to offer my saas customers the ability to add their custom domains within the dashboard on certain routes like "/dashboard"
So i set up a under pages - > custom domains > add a domain i.e. custom.mycustomer.com which points to my pages.dev
And my customer adds a dns CNAME setting of custom to pages.dev
This all works!
But it works for all routes, we need only custom domains on /dashboard.
Do i need to do something else what are my options?
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