Do you have any idea what "itemid" param stands for?
Tried to put assetId, but no success
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Nevermind, I was using wrong SteamId, after putting correct one, splitting works as expected.
You can use asset id as itemId.
Interesting is that you can split only 1 item from stack at once request, so if you have 10 items, you will need 9 requests to separate them.
Thanks you so much, you saved a lot of my time!