1 Answer
You can't unlock it in template-based emails. You instead have to edit the template directly.
Release notes from when my team added this feature can be found here https://help.salesforce.com/articleView?id=mc_rn_january_2018_content_builder.htm&type=5
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1Do you have a reference for this claim? Adding a citation or a bit more detail would strengthen the quality of this answer.– Adrian Larson ♦Commented Dec 11, 2018 at 17:16
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Please edit your answer with any clarifications. That is not what comments are for. It would be best if you also include any relevant content here.– Adrian Larson ♦Commented Dec 12, 2018 at 18:12