How to use command line completion in Midnight Commander?

You just need to prepend it with Esc: Esc-Tab does completion, and it will even give you a tiny dropdown if you do it twice. (That being said, you probably won't get the more fancy expansion possibilities of some shells.)

There was a fix in MC 4.8.19 which makes redefining tab work

https://midnight-commander.org/ticket/3672

This config sample from the fix makes simple Tab work:

[main:empty-cmdline]
ChangePanel = tab
[main]
ChangePanel =
[input]
Complete = tab; alt-tab