Take a look at this link with instructions how to add that to the HTML in your campaign. You will need to add <strike> tags into the HTML around the text that you want to have the strikethrough.
To get to the HTML of your text block, click your cursor into the text block and click on the </> icon in the tool bar. Then you will want to add the opening <strike> tag in front of the text you want to have strikethrough and the closing </strike> tag at the end.
That should do the trick! Feel free to give me a shout here if you have any questions.