post {protocol}://api.ws.sonos.com/control/api/{version}/groups//playback/seekRelative
Use the seekRelative command in the playback namespace to seek to the a relative position in the current track. Optionally add the cloud queue itemId to target a specific track.
Use itemId to ensure the group seeks on the current item. If the current item’s itemId does not match the one provided, the player returns ERROR_INVALID_OBJECT_ID.