It's now on the post added it while you were posting.
You may have seen other attempts with limited success or just a full stop, I've been trying various things and deleting failures, some worked but were too complicated, then the obvious hit me, the http or https:// tells the system what to do, without it unfurl="TRUE"] is not added and the link still opens.
From XF manual When a URL is inserted into content and that URL exists on its own line within the content, we will "unfurl" it to a richer preview which includes the page title, metadata logo, description and favicon. Such rich previews add more context for users as to what the link contains. URL unfurling can be used anywhere that accepts BB code currently.
The functionality is enabled by default, but you can switch it off if you want to in the "Messages" section of the Admin CP.
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