How to reach specific origin behind LoadBalancer

I have 3 origin VPS behind Loadbalancer.

Sometimes I need to reach to a specific origin to test new code deployments other times We want to keep the developer to reach to a specific region while under development and all the live traffic should go to other 2 origins.

Direct IP access is not possible as this is a SaaS product and need to rely on the HOST header sent from the browser.

Is there any parameter we can use in the URL to reach to a specific origin?

Our developer are around the world, Loadblancer is setup a dynamic Steering.

Any help in this matter please?

Thanks,
Erfan

As some one suggested that we can probably use Chrome extension “ModHeader” With the IP address.

But the thing without setup is that Firewall is configured on the Origin servers to only allow CloudFlare IP address.

We cannot whitelist all the developers IP address as mostly don’t have static IP address at all.