Currently, you can only add one line of text in the back end of the product builder, you can not hit enter and start a new line within the same text builder. I have signs that have multiple lines of text and I only want the customer to be able to change the color and font of the entire paragraph, as opposed to each individual line.
On the front end when typing you can hit enter and start a new line within the same text builder. You should be able to do this on the back end. Maybe on the back end add the code <br> to tell the text to start a new line within the same text builder?
Just double-click on the text element in the product stage. That allows to enter paragraphs.