Stringify JSON in Logic App

Do you need the stringified message to include opening and closing double quotes?

I've tried this and it worked for me.

1. I have my JSON object as an output of a compose
2. Then, I initialised a variable with the Base64 encoded value of the escaped stringified JSON (you need to add ALL the proper escaping required,mine was just a PoC)

3. Then, you send the variable already in Base64 to Service Bus. (You need to remove the encoding on that action).

   "actions": {    "Compose_JSON_Object": {        "inputs": {            "message": "I want this as a string"        },        "runAfter": {},        "type": "Compose"    },    "Initialise_Variable_with_Stringified_JSON_Base64_Encoded": {        "inputs": {            "variables": [                {                    "name": "jsonAsStringBase64",                    "type": "String",                    "value": "@base64(concat('\"', replace(string(outputs('Compose_JSON_Object')), '\"', '\\\"'), '\"'))"                }            ]        },        "runAfter": {            "Compose_JSON_Object": [                "Succeeded"            ]        },        "type": "InitializeVariable"    },    "Send_message": {        "inputs": {            "body": {                "ContentData": "@variables('jsonAsStringBase64')",                "ContentType": "text/plain"            },            "host": {                "connection": {                    "name": "@parameters('$connections')['servicebus']['connectionId']"                }            },            "method": "post",            "path": "/@{encodeURIComponent(encodeURIComponent('temp'))}/messages",            "queries": {                "systemProperties": "None"            }        },        "runAfter": {            "Initialise_Variable_with_Stringified_JSON_Base64_Encoded": [                "Succeeded"            ]        },        "type": "ApiConnection"    }},

This way, I got the message stringified.

HTH