You could just create a rectangular aperture in the Solder mask layer so that the copper text and some area round it is exposed. Or you could just create an aperture in the Solder mask layer using a copy of the text itself: 1. Place your text in the Top (or Bottom) copper layer. 2. Copy and paste it in exactly the same position but do not deselect it. 3. Move it to the Top (or Bottom) solder mask layer.  4. Increase the line width to a little greater than the size of the text that you want. 

Pads are automatically exposed copper. You could also try: Create your text then export it as DXF; Import > DXF into the Top Layer; Select, right-click > Convert to Board Cutout; Then Select the Board Cutout (not the vertices); In the upper right hand panel, select Solid (should default to Top Layer); Then Select the Solid Region (not the vertices); Right-click > Convert to Pad.

@andyfierman Thank you these instructions were really helpful.