Rewrite domain and keep subdomain

[Matt .]

Hi Guys,

I'm figuring out how I can rewrite a domain in a req.http.Host

Like I type in my  addressbar: subdomain.myFIRSTdomain.tld

Where Varnish needs to catch this using req.http.Host filter out the
domainname and make req.http.Host actually:

subdomain.mySECONDdomain.tld

I'm figuring out how to do so, but I can't find the right way/docs.

I hope someone can help me out !

Thanks,

Matt