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Open the browser development tools. Hotkey for most browsers is F12.
Select the console.
On the command line, enter :
Code Block srv.openModal(true,'<FORMKEY>')
The <FORMKEY> can be found in the URL of the preview page. See How do I open a Mopinion feedback form with my own button or link? for more details on retrieving the <FORMKEY>.
An overlay should appear on the webpage.
If your tag does not contain the reference to pastease.js, then you are probably running an older version of a Mopinion tag (for example mopinion.min.js). In that case, you can trigger it this way:
Open the browser development tools. Hotkey for most browsers is F12.
Select the console.
On the command line, enter:
Code Block Mgo.Mopen()An overlay should appear on the webpage.