Sharethrough offers DSP-initiated User ID Syncing for the exchange. Sharethrough hosts a match table so you can request previously sync'd IDs at bid request time.
In order to sync a user, call the following as a pixel, or as the endpoint directly. Both http/https are supported.
<img src="https://match.sharethrough.com/sync/v1?source_id=[DSP Source ID]&source_user_id=[DSP User ID]" width="1" height="1">
Replace the following parameters:
DSP Source ID should be replaced with your given seat key / source ID
DSP Uer ID should be replaced by the User ID you'd like us to return at bid request time.