curl --request POST \
  --url https://api.example.com/admin/category/children/assign \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "newParentCategoryID": 123,
  "forTheseCategoryIDs": [
    123
  ]
}
'

{
  "success": true,
  "data": "<unknown>"
}