curl -X POST 'https://api.sendforsign.com/api/placeholder' \
     -H 'X-Sendforsign-Key: YOUR_API_KEY' \
     -H 'Content-Type: application/json' \
     -d '{
    "data": {
        "action": "update",
        "clientKey": "YOUR_CLIENT_KEY",
        "contractKey": "CONTRACT_KEY",
        "placeholder": {
            "placeholderKey": "PLACEHOLDER_KEY",
            "name": "Name",
            "value": "Value"
        }
    }
}'

{
    "placeholder": {
        "id": "1",
        "createTime": "2024-03-14T20:34:08.000Z",
        "changeTime": "2024-03-14T20:38:39.000Z",
        "name": "Name",
        "value": "Value",
        "type": "1",
        "placeholderKey": "PLACEHOLDER_KEY",
        "position": null
    },
    "code": "201",
    "message": "Placeholder updated"
}