how to access internet server behand F5 by a local address on F5,observed portforward ?

Didn't understand your query fully but below may answer your query:-

1st option) Do you have route towards internet Router/FW from your backend server?

2nd option)if you are looking f5 to be use as reverse proxy , server able to connect to some website which have static IP.. you can create VS and backend as public ip ...also f5 self ip as source in firewall and destination as public ip