Searching messages for content contained in “Message Source”?

Anyone know a way to search messages for content contained within the Message Source of messages, using built-in methods or third-party extensions?

When I search for content contained within the Message Source of messages, a Body search will not find those messages. Here is the list of criteria choices available when I perform a search. I’m not sure if “Customize” would be useful for what I’m attempting. This is Thunderbird 102.12.0 (64-bit) on Win10x64:

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