There are several categories of stickers that can be added to your slides. You can choose a category by selecting the name which will expand the box to show you the available stickers.
One you find the sticker you like, simply click on it to add it to your slide. Once it has been added, you can click the settings icon to change the fill color or the border color.
If you are looking to create a border effect with dashes or dots, you can also use the Shape App and use the Rectangle shape and go to settings to change the border style.