Please use 4 back slashes to print a single back slash in the resultant JSON string. Two slashes turns into one slash and escapes C#'s string only, to escape JSON, you will need another two. Or use can use the @ string for simplification.
"{"test123 ": "?'?u00141u0019?\\±y?± u0013Eú", }"
Or
@"{""test123 "": ""?'?u00141u0019?\±y?± u0013Eú"", }"
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