According to Helen's post, you can get it by watching the stream. Read the post:
Get Your Free Nekros Prime and Lotus Ephemera
For the first time ever, we’re streaming all Main Stage TennoCon panels throughout the day before TennoLive! Watch any panel on Twitch or Mixer for 30 consecutive minutes to receive a free Lotus Ephemera. This drop ends at 5:59 p.m ET.
Then at 6 p.m. ET, watch TennoLive 2019 and witness the full reveal of Warframe: Empyrean to get a FREE Nekros Prime! You’ll earn him by watching the show for 30 consecutive minutes between 6 p.m. and 7 p.m. ET. (Includes a free Warframe slot!)
Want a reminder for TennoLive? Like this Tweet for a scheduled reminder the day of the event!