Sending String to API request through variable

So I have a custom action that is returning String value (some text). I have API call, that is a POST request and has a variable that needs to be nested into JSON body. For example:
{ "first": "blabla","second": "THAT VARIABLE"}

I have an issue because when I send that variable to API call it has "" that is causing error since JSON body already has "". But if I try to remove "" that are around variable, I cannot save the API because JSON format is not OK.

Does anyone know what I can do?

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