When filling the survey at http://LEGOsurvey.com/DCSV you get a code in the end. I am unsure if it's always the same code for everyone or if there are several spread randomly... i'll just drop mine in here. :)

If you get any code from the survey that is not listed here, just drop it into the comments and i'll add it to the list in time.

Originally posted by Samakin on 20181017:

You should note that using codes to unlock characters does not add to the 100% count, and so you use one code, that save game is not viable to get the 100% achievment.

I can not confirm nor deny this information yet, as i have not reached the end so far. Will update once i know more, as i used my survey code. So if i lack one unlock, that's the reason i guess. :)
As of now (20181021), i can confirm that putting in one code does still give the option to unlock the "All together now" achievement and get 100%. (proof on https://test-steamproxy.haloskins.io/id/autositz/stats/appid/829110/achievements - i only have one savegame no restarts/deletes)

• E6HUY: Plastic Man

Provided by "Whitedust Alchemist" on 20181017

• Adam Strange — QF7NT
  
• Atrocitus (Red Lantern) — 9RHJJ
  
• Black Canary — ND6AL
  
• Blue Beetle — 8VV5Q
  
• Clock King — Z6AU7
  
• Detective Chimp — BQWSG
  
• Dex-Starr (Red Lantern) — KFLQM
  
• Doctor Fate — KPTCC
  
• Doctor Light — 6NWX5
  
• Doctor Poison — GWWBS
  
• Firestorm — HFMHM
  
• General Zod — D8577
  
• Green Lantern (John Stewart) — KNJ2P
  
• Hawk Girl — LNSB9
  
• Jessica Cruz — S5DB6
  
• Lady Shiva — XQP2L
  
• Martian Manhunter — Y7MFR
  
• Mister Mxyzptlk — NURPU
  
• Monsieur Mallah — GM9MX
  
• Ravager — UVWHS
  
• Red Robin — JNLPY
  
• Red Tornado — CKDRF
  
• Star Sapphire (Violet Lantern) — QD2GY
  
• Terra — F79GU
  
• The Trickster — YRZMS
  
• Toyman — VB5AS
  
• Clayface — YUCOP3 (?)
  
• Harley Quinn — PLG1Z (?)