Embed custom call-to-action messages in each page you share

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"Sprdin is a link shortening tool that allows you to advertise on any website that you share. Engage with your audience, even when you are directing them to another site with customizable banners and alerts. Add call to actions, signup forms, and images to the alerts that can coincide with your branding through colors, logo, and imagery. Every link you share is an opportunity to bring traffic back to your site."
Interesting idea & execution -- looks similar to @Quibb's header bar. But how do you deal with sites that don't don't allow iframing? (github, google, etc...)