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I'm guessing this one:

Apl Functional Symbol Tilde Diaeresis
Miscellaneous Technical
U+2368⍨

To get it, I went to https://unicode-table.com/en/search/?q=U%2B2368 and just clicked copy.

Huh! I would not have ever thought something that simple would work. Thanks!

Inserting ⍨ works to insert the symbol on a web page. Thanks for the link that gave me the numerical value and showed that no spaces are required.