"when" : [
    {
        "blockType":"when",
        "blockOptions":{
            "method":{
                "name":"compareStrings",
                "args":{
                    "item":"item",
                    "comparator":"comparator",
                    "value":"value"
                }
            }
        },
        "fields":[
            {
                "name":"item",
                "type":"item",
                "value":"5de64f6a70c7be0541cc0853"
            },
            {
                "name":"comparator",
                "type":"string",
                "value": ">"
            },
            {
                "name":"value",
                "type":"string",
                "value": "<value>"
            }
        ]
    }
]

In a case of expression, the field should reference the existing expression by name:

				
					{
    "name":"value",
    "type":"expression",
    "value":"<expressionName>"
}