Have a look in the Guru Documentation page at Front Page 1 and you’ll see the Plugin and exact code they use in the demo – https://appfinite.com/theme-setup/guru-theme-setup/#fp1
Make sure to download and install the plugin in order for the popup to work. You can also use other plugins if you want.
Also, which type of widget are you using? Every since the WordPress update, the basic “Text” widget has changed. So now you have to click the “Text” tab when adding code……Or you can use the Custom HTML widget which allows you to add code