You can't.
The camera triggers the flash optically. No light = no signal = no trigger.
However, it actually operates on the IR spectrum, so if you really wanna cut away the light from your built in flash, what you can do is to find a developed negative and put it in front, it will block most of the optical spectrum while the IR spectrum will be able to pass through and trigger your F42AM (as well as other Sony/Minolta flashes).
But if you don't want to go through all these troubles, the (next) best alternative would be to buy a radio trigger.
There are a few types and you might want to google these to find out more before you make your purchase.
-PT-04
-CTR-301P
-RF602
-RF603 (new!)
-Pocket Wizards (the mother of flash triggers)
And sometimes reading your manual helps...
All the best!