Select your site from the My Sites dropdown at the top of the page.
Under the Prompts section, you'll see a Post Share Prompt option. Use the togglebutton to turn Post-Share Prompt on / off:
Click Configure to access additional customization settings, like changing the Header text, adding your Social Media follow buttons, and enabling/disable ads. You will see a live preview of your Post-Share window on the right-hand side.
To enter a custom header, just replace the default 'Thanks for Sharing' text with your desired text.
Use the toggle buttons next to the Follow Buttons and Post-Share Ads sections to enable/disable those features. Use the gearicons to access additional customization settings.