Not sure if this helps, but in our site we synchronise products from another sytsem, so this is what we do to add the products(child items) to the shop (parent)

 var newproduct= contentService.Create("this product", parentid, "product");

    you can then set properties for newproduct using

    newproduct.SetValue("attributename",value);

    and the finally do

    contentService.SaveAndPublish(product);

Hi Reddick, Thank you for your response. It's working when adding child to parent but actually I am trying to add value to nested content property programmatically in document type page. I am having a bit of trouble getting it working fully.

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