https://docs.microsoft.com/en-us/dotnet/api/system.text.json.jsonwriteroptions.indentcharacter?view=net-10.0
JsonWriterOptions.IndentCharacter Property (System.Text.Json) | Microsoft Learn
Defines the indentation character used by Utf8JsonWriter when Indented is enabled. Defaults to the space character.
property systemtextjsonmicrosoftlearn