System.Text.Json - The JSON value could not be converted to System.String

I have the following json being sent to api

{
     "Id":22,
     "UserId":22,
     "Payload":{
        "Field":"some payload value"
        ...more unknown field/values go here
     },
     "ContextTypeId":1,
     "EventTypeId":1,
     "SessionId":1
}

I would like to map it to the following:

  public class CreateTrackItem : IRequest<int>
    {  
        public int Id { get; set; }
        public int UserId { get; set; }

        public string Payload { get; set; }
        public int ContextTypeId { get; set; }
        public int SessionId { get; set; }
        public int EventTypeId { get; set; }
    }

When mapped the Payload property fails that it cannot map json to string, i simply want the Payload to be a string version of the json (will go into a jsonb field in postgres)

I am using .NET Core 3.0 and prefer to use the built in System.Text.Json if possible before switching to Newtonsoft.

You can use Object type instead of string. Or use JToken type from Newtonsoft as Ryan already commented above.

public object Payload { get; set; }

 public class CreateTrackItem : IRequest<int>
{  
    public int Id { get; set; }
    public int UserId { get; set; }

    public object Payload { get; set; }
    public int ContextTypeId { get; set; }
    public int SessionId { get; set; }
    public int EventTypeId { get; set; }
}