There are various ways to do it and partly would depend on the size and weight of what you were trying to attach to the metal post. If the clay is lightweight and has no parts that stick out so that they could put real stress on the bond, you can just glue it to the flat metal base with E6000-Goop, or maybe even any kind of glue though I might sand the metal a bit first (for any glue, wipe with alcohol to remove any grease).
If the clay is heavier or larger though or has parts that dangle or stick out so there could be more stress on them, you can glue the baked clay on with a 2-part epoxy glue (or possibly E-6000) --as mentioned, making an impression of the metal into the raw clay before baking it can be helpful in any of these cases.
You could also glue the raw clay to the post with a bit of liquid clay then bake them together (liquid clay isn't tacky
though, so a touch of superglue next to the liquid clay would help hold the clay in position on during baking).
Best of all though would be to encase
the base part of the metal post inside the back part of the raw clay for the absolutely-most-secure bond (the clay-metal unit would then be baked together... there's no problem in baking metal with polymer clay). That could be done in several ways.
If you're interested, the Jewelry page at my site has info on making post earrings with polymer clay (as well as other earrings):http://www.glassattic.com/polymer/jewelry.htm
...click on Earrings
, then scroll down to Post Earrings
...and also click on Pins
for in a similar encasing method that's done with a strip or strips instead to hold pin backs
securely to the clay, which could be done with post earrings as well
P.S. I tend to think of "stud" earrings as being posts with a ball (stud) on the end, though I may be wrong... some stud findings also come with a metal loop at the bottom for adding a dangled bit. At any rate, if you have a "stud post-type" earring you want to add clay to, you could surround the metal ball (at least a little more than half-way around it) with the clay you want, then bake together --there would be no glue necessary there because the clay would create a mechanical
hold, like the other encasing method would.