Currently the IMI Documentation shows no way of reading or editing the custom device attributes of a ThinClient. This is indicated to be available / possible in the UMS Console (see attached screenshot)
It could be the attribute ‘deviceAttributes’ in the response when requesting ‘https://[server]:[port]/umsapi/v3/thinclients?facets=details’ but the value of this attribute is empty even when there are custom device attributes set on the device (see screenshot)
Update: Documentation has been added for reading the deviceattributes (https://kb.igel.com/igelimi-v3/en/get-v3-thinclients-tcld-facets-deviceattributes-60983400.html) but not yet for writing them to the UMS (https://kb.igel.com/igelimi-v3/en/put-v3-thinclients-tcid-2723464.html).
It is however already possible to do so: https://igelcommunity.slack.com/archives/C8FC01UNM/p1654003603132039?thread_ts=1651072479.559879&cid=C8FC01UNM