dorsetdigital

Do you mean that you want to be able to have nothing selected?

dorsetdigital

Then you just need to add the right code to your page template. Where that goes specifically will depend how the site is built, but it will be in the templates directory, possibly in http://Page.ss or maybe in a footer include or similar

dorsetdigital

You need to decide how the popup is to be triggered (bear in mind, automatic pop-ups are hated by most web users)

dorsetdigital

OK. https://getbootstrap.com/docs/4.3/components/modal/ Bootstrap can provide the pop-up, etc. out of the box

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getbootstrap.com  
Modal

Use Bootstrap’s JavaScript modal plugin to add dialogs to your site for lightboxes, user notifications, or completely custom content.

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dorsetdigital

Is the site built using a front-end framework (eg. Bootstrap, Foundation, etc.) ?

dorsetdigital

The pop-up and content is mostly a front-end job in that case. It's some HTML code, Javascript / CSS to create the popup. It's not really SilverStripe-specific.