if you just want to change http host header and uri in request, you can use HTTP::uri.
Redirects, Rewrites and App Transfers via iRules by Colin Walker
https://devcentral.f5.com/articles/redirects-rewrites-and-app-transfers-via-irules
if you also have to rewrite domain back (e.g. from domain to benefitsmailbox.cmi.org), you may have to check proxypass irule or rewrite and html profiles.
ProxyPass v10/v11 by Kirk Bauer
https://devcentral.f5.com/wiki/iRules.ProxyPassV10.ashx
Manual Chapter: Configuring the BIG-IP System as a Reverse Proxy Server
https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/ltm-implementations-11-4-0/21.html