MDI Call Sub

This is probably going to sound like a really dumb question - how do you execute an O-sub routine from MDI?

I'm assuming it can be done but I've never found it in the manual, and I've never come across it in the forum!

only external sub can be called

no internel g-code

www.linuxcnc.org/docs/html/gcode/o-code....#ocode:calling-files

On my machine i can call it from MDI the same way as call it from a .ngc routine just entering

Thanks guys - I thought it would be something simple.

I've even used that exact call in one of my own O-subs, but the limited capacity of my brain couldn't connect the dots!