To get the plate thickness, you would actually have to know if the load pointing down on the top flange was caused by another beam running across the top of our beam, or if it was due to a vertical column sitting on the top of the flange.
We will assume that it is a beam going across our beam. Design the plate thickness just like you would if our beam were pressed up into a plate embedded in a concrete ceiling. I.e. get a reasonable plate size, at least one dimension of which extends across the width of the top flange of our beam and no shorter than the flange width of the crossing beam (you could assume the plate is square), unless you need a longer value for N from web shear/crippling/etc.
Then divide the load coming into our beam by the plate area to get the pressure between the top flange of out beam and the plate, and design the plate thickness using equation 5.13 on page 256.