Converting Java Object to Camunda Variables

Hi,
I want to send a post request to start an instance of process using Spring’s WebClient.
This request only accepts input as defined in variables like this

{
    "variables": {
        "userName": {
            "value": "Gagan",
            "type": "String"
         }
    }
}

while this is what I get as a request

{
    "userName": "Gagan"
}

Currently I have created a Mapper to convert the request into variables like structure. I wasn’t able to understand or make it work using the Spin library maybe due to time shortage. But I’m really interested in knowing a better way to do this. Creating own mapper or something is already provided that I can use.