The card widget is the largest format with the immersive, which means that unlike the bubble or square widgets there is no Play icon, there will be an icon of its own on the cards:
However, if you want to add an icon play here are the steps to follow:
From your story edit, click on "export" at the top right and export the page that will be the cover page.
Using an image editing program, add the icon play of your choice.
Click on "Publish" at the top right and then on "Story Settings".
On the right hand side you will see a 'Replace' button which allows you to replace the cover image used in the widget.
Replace the current image with the exported image.
Republish the story.
💡 Tips and Tricks: You don't have to add an icon play on all stories, only on the cover of your widget, i.e. the first page of your stories.
You can find icon plays on any search engine with "flat icon play button" you can also ask internally to match your site branding.