Hidden pre chat values in enhanced chat embeeded service deployment on messaging session.

Hi all and all the best to wherever you are,  

i run out of ideas regarding passing hidden pre chat values from embeeded service deployment (enhanced chat) to a messaging session record. I setup everything in messaging settings, the custom parameter, mapped it in parameter mappings, i use the exact same flow variable, which is of same data type and available for input. In the flow i have an update element which should write the pre chat value to the messaging session before routing the session to the bot in the Inbound Flow. 

In embeeded service deployments pre chat is active and the hidden field is selected. I added according to documentation the method.  

"window.addEventListener("onEmbeddedMessagingReady", () => { 

    embeddedservice_bootstrap.prechatAPI.setHiddenPrechatFields({ 

        "preChatWebsite": "Austria" 

    });"  

But the value is not passed. However the flow itself fires. So i can write hardcode "test" into the field on messaging session object via flow everytime i start a conversation. It as well does not get overwritten from another flow. I test this in dev console with a VF page.  

Is there something i am missing? Do i have to create a case first maybe? 

Would be happy for help here. Thanks a lot in advance.  

Best