I found that for now there is a workaround only in Stocksubmitter+:
"For a few months it was impossible to submit content to #iStock due to the latest changes in attempt to protect themselves from their own contributors. However, we were able to find a workaround. To submit your content to this agency, please open the Manage Agencies section, find iStock ESP and download the browser extension from there.
We didn't test the extension in the browsers other than Chrome, but it may work as well.
Unfortunately, there is no solution for StockSubmitter yet.
Please let us know of any issues you encounter while using the extension."
For Stocksubmitter developers replied: "we will add support for SS with extention later, now only M+"