On a Mac, highlight what you want. Depress "Command" and then "c" for Copy, then click on where you want to paste it and depress "Command" and "v" for Paste (maybe the v is just easier to reach than the p).
And you can add a link by clicking on the little icon about 2/3 of the way to the right-hand side that looks like a mountain range and a sun.
Correction: SJ Baty is right. Use the icon that looks like a strange sideways paper clip (or a link from a chain!), not the one that looks like mountains and a sun.