Can you help me create a link in Drupal 8 using the ccs class?

Question

css

Installed the Simple Popup Blocks module. It allows you to call the block as a pop-up window by clicking on the button.
Link to the module: https://www.drupal.org/project/simple_popup_blocks
The module has instructions on how to make it clickable so that this window appears.
Item 7 says that you need to “Provide a CSS selection source. This is the CSS selector that the user is about to click."
And in point 9: “Go to the popup page and click the select CSS button. This will now bring up a pop-up window."
Question: how to create this css selector so that it can be put as a link to the button that triggers this popup?

Answer 1

So here in the instructions it is written in a simple way, you just need to find in the html code that piece of code (which is the popup) that has a certain class / id and register it.