How to assign a quote escape character in assign variable

Content

The Assign variable expression is "userName sw " + $page.variables.userNameTxt

In JSON, it says 

"parameters": {
            "$variables.filterQueryStr": {
              "source": "{{ \"userName sw \" + $page.variables.userNameTxt }}"
            }

It works if the userNameTxt is of Numeric, but if the userNameTxt is of String then it has got syntax error

From Postman, having the extra \" works but where as it does not work from VBCS

"filter : "username sw \"Test\"",

I have difficulty in concatenating variable with a static string with quotes. Please help on how to frame this expression in VBCS Assign variable