Just create it on a new layer.
> Create a colour block using the shape tool with the required colour selected and change the transparency of the shape layer to 50%
> above this layer type the words you want on it
> hold control and click on the text layer creating a marquee for the text, then invert the marquee (ctrl+shift+i)
(before ctrl+shift+i is selected)
(after that action)
> hide the text layer and select the shape layer and create the mask using the mask icon on the layers pallet
should look something like...
I hope this helps?