We have configured extended choice parameter with  JSON Parameter Config Groovy Script option such that after selection the value of parameter field is like that : load_test_type=

{"LoadTestType":"Iterative","MaxThinkTime":5,"MaxUsers":1,"MinThinkTime":2,"NoOfIteration":1,"RampUpTime":1}

now we wanted to call the same with build with parameter rest api as:
/buildWithParameters?cloud_team_name=load-and-performance-other-team&load_test_type=

{"LoadTestType":"Iterative","MaxThinkTime":5,"MaxUsers":1,"MinThinkTime":2,"NoOfIteration":1,"RampUpTime":1}

this  call have not populated any value in the field , refer screenshot for how we take input for the same :

same groovy json written for one option:
 schema: {
    "title": "",
    "oneOf": [
     

{         "title": "Select",         "type": "null"       }

,
      {
        "title": "Standard: Iterative",
        "type": "object",
        "properties": {
          "MaxUsers":

{             "title": "Maximum users",             "type": "number",             "default": 1,             "propertyOrder": 1           }

,
          "RampUpTime":

{             "title": "Ramp-up time (in sec.)",             "type": "number",             "default": 1,             "propertyOrder": 2           }

,
          "NoOfIteration":

{             "title": "Iterations count",             "type": "number",             "default": 1,             "propertyOrder": 3           }

,
          "MinThinkTime":

{             "title": "Minimum think-time or wait-time (in sec.)",             "type": "number",             "default": 2,             "propertyOrder": 4           }

,
          "MaxThinkTime":

{             "title": "Maximum think-time or wait-time (in sec.)",             "type": "number",             "default": 5,             "propertyOrder": 5           }

,
          "LoadTestType": {
            "type": "string",
            "default": "Iterative",
            "options":

{               "hidden": true             }

          }
        },
        "required": [
          "MaxUsers",
          "RampUpTime",
          "NoOfIteration",
          "MinThinkTime",
          "MaxThinkTime",
          "LoadTestType"
        ],
        "additionalProperties": false
      }