You can use below code and add following section in task.josn file to achieve this:
"inputs": [
{
"name": "rootArea",
"type": "pickList",
"label": "rootArea",
"defaultValue": "",
"required": false,
"helpMarkDown": "Select the root area.",
"properties": {
"DisableManageLink": "True"
}
},
{
"name": "childArea",
"type": "pickList",
"label": "childArea",
"defaultValue": "",
"required": false,
"helpMarkDown": "Select the child area.",
"properties": {
"DisableManageLink": "True"
}
}
],
"sourceDefinitions": [
{
"target": "rootArea",
"endpoint": "/$(system.teamProject)/_apis/wit/classificationNodes/areas?$depth=2&api-version=1.0",
"selector": "jsonpath:$.name",
"keySelector": "jsonpath:$.name",
"authKey": "tfs:teamfoundation"
},
{
"target": "childArea",
"endpoint": "/$(system.teamProject)/_apis/wit/classificationNodes/areas?$depth=2&api-version=1.0",
"selector": "jsonpath:$.children[*].name",
"keySelector": "jsonpath:$.children[*].name",
"authKey": "tfs:teamfoundation"
}
],
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