我正在

I was having similar issue, and have found a solution, see below the code and comments. I am using Newtonsoft though but it is worth checking if you can use Newtonsoft library and have not found a solution for System.Text.Json.

JSON中的所有控件都是组件对象/数组的一部分，因此我们可以使用JSONPath，有关更多信息，请参见链接.

All controls in your JSON are part of component object/array so we can use JSONPath, see the link for more details.

public void IterateJson(JObject obj, string mandatoryFieldKey)
        {
JToken jTokenFoundForMandatoryField = obj.SelectToken
("$..components[?(@.key == '" + mandatoryFieldKey + "')]");   
            //Now we convert this oken into an object so that we can add properties/objects in it
            if (jTokenFoundForMandatoryField is JObject jObjectForMandatoryField)
            {
                //We check if validate already exists for this field, if it does not
 //exists then we add validate and required property inside the if condition
                if (jObjectForMandatoryField["validate"] == null)
                    jObjectForMandatoryField.Add("validate", 
JObject.FromObject(new { required = true })); //add validate and required property
                else
                {
                    //If validate does not exists then code comes here and 
//we convert the validate into a JObject using is JObject statement
                    if (jObjectForMandatoryField["validate"] is JObject validateObject)
                    {
                        //We need to check if required property already exists, 
//if it does not exists then we add it inside the if condition.
                        if (validateObject["required"] == null)
                        {
                            validateObject.Add("required", true); //add required property
                        }
                    }
                }                
            }  
}