how do i force iis to use a specific value for the http host header during a rewrite?

try to set the preserveHostHeader to true by following the below steps:

1)open IIS manager, select the server node.

2)double clic configuration manager.

3)from the section drop down select system.webServer/proxy

4)set preserveHostHeader to true

Note: if you are trying to change the request header it is not possible by using iis URL rewrite rule.