MMS Proxy — Definition

An MMS proxy is an intermediate server some carriers route picture messages through, specified by a host address and a port number in your APN settings.

Not every carrier uses one. Plenty leave the MMS proxy and port blank, and on those networks a blank field is the correct setting, not a missing one.

In practice

If your carrier's page shows an MMS proxy and port, enter both exactly. If it shows them blank, leave them blank. Inventing a value to fill an empty field is a common way to break picture messaging that would otherwise have worked.