Eagle PCB design how to make flat pins for the power socket
Two options:

You don't.  Use the smallest round hole that the flat pin is guaranteed to fit thru.

Specify a slot in the routing or maybe dimension layer.  However, contact your board house first to find out how to specify the slot must be plated, which layer they want to see what in, etc.  Be prepared for extra cost.



I've always used option 1 so far.