To create an Onclick popup, please, follow the steps bellow:
1. Go to the Pages/Posts section of your Dashboard
2. Choose the Page/Post you want to add an Onclick popup
3. Click "Insert popup"
4. From the box, select the popup you need to open Onclick
5. Choose the load type, select "Click"
You should enter a text (like "Click here") inside your popup shortcode [sg_popup id="1" event="click"]Click here[/sg_popup] so that a popup will open when clicked. (enter your popup id number "1" is an example: sg_popup id="1")
After updating the page you can preview it and see the following page:
Here's the popup opened after you click on your "Click here" text:
That's it! Now you'll have an Onclick popup on your page/post!
And here you can see an alternative method to open an Onclick popup via CSS class: How to trigger an Onclick popup via CSS class?