When testing a new prompt, you may already have a set of examples you want to test against. Assuming you have already created a dataset for these examples (see Datasets if you still need to), you can easily add these examples to the playground. In the playground, after you click Add test case, a drawer will open, allowing you to select any of your datasets to add to the playground. After selecting a specific collection, you can check all the examples you want to add.
The column names from the datasets should align with the template variables in your prompt.