.htaccess RewriteRule?

Hi! Since i moved my multisite installation to another domain my preview-URL doesn’t work. I’ve noticed that it does however work if i change the URL manually from

http://mydomain.com/?p=706&preview=true
to
http://mydomain.com/index.php?p=706&preview=true

So a solution would be to add a rewriterule to .htaccess. Im not familiar with the syntax though, so i wonder if anyone know how to write this rule?

Im grateful for any tips or solutions!