Is it possible to transform a microwave transformer into a current measuring transformer?
Leave the primary coil intact, you might as well use it as the output winding.
Make sure all other windings and the magnetic shunts have been removed.
Thread a turn or three of insulated wire through the core to form the primary (I assume you want to step the current down). Use wire thick enough for your projected current.
A MOT core is not ideal for measurement applications. It has been designed to save the very last penny in manufacturing costs, for running at the kilowatt level with forced air cooling. It's likely to have permeability lower, and losses higher, than would be ideal for a current measurement transformer. But you can expect it to work to some extent.