Hi Annother, Thanks for contacting us!
You can copy the symbol from here: © and then paste it into the text area in Ribbet. Note however that not all fonts support the copyright symbol, but many do including the “Basic Fonts” at the top of the font list. You can add your text using any font, and then add a second piece of text for the copyright symbol in a font which supports it.
Let us know if you have any further questions!