You've just created a SPEC AD! You have a simple white piece of paper.
At this point, you may be wondering if you need to create a full color print ad so it looks like what you would see in a magazine. Copywriters are not expected to be graphic designers.
Your talent lies in writing and the copy is what a client/employee is going to be analyzing when interviewing you, not the design. If your copy isn't strong, it's not going to matter how many pretty colors and pictures you put into the design. Focus on your copy.
Even experienced copywriters have basic text ads in their portfolio. Many copywriters update their portfolios to include their latest projects. These projects may not be in final printed form so all they have to show off is the text they wrote. So don't shy away from text and get caught up in the lack of visuals.
However, you can dress up those text ads to make them stand out in your portfolio.