I have trouble getting S- up ranks as supports. I tried going a carry as a support and it kinda works but I don't like it.
I main thresh and I always find it hard to get S-.
Is there a certain way to get S- as a support? Maybe i'm doing something wrong.
Thanks guys 
I was able to achieve an S- as a support Nautilus before the Mastery tokens were released. Frankly being a support carry is hard enough if you're not a mage support willing to throw the wards, rely on the utility, produce a high damage output with a KDA, and whatever other elements fall into place when determining the score.
Therefore I would just say to do something unconventional outside of support role to get the S ranks. I did that for Taric particularly through jungle and top gameplay.
You need to place a lot, and I mean a lot of wards. at the end you should average around 1.2 wards per minute of game time. Participate in at least 23 kills with very few deaths and assist with dragons and towers.
All ratings are in comparison to how well others have done with that champ in that role.
A good way to tell the qualifications for a -S or higher is to play a game and do your best, look at your rating. If you got a A+ your last game with of KDA of 1/1/10 then push to get more assists, 1 extra kill, or prevent that death next time. Cs is also factored into ratings. Eliminating enemy vision counts towards Cs too.
Glhf
3/3/21 for example, with 40 farm. That should get you it. You may not need the farm though...
Ward lots. Deward lots. Play a significant role in fights. Roams help. Kills and cs aren't all that important (you're a support, afterall!), though it doesn't hurt to ensure you can cs when your adc is away.
place a lot of wards, the more wards the higher the grades
IMO, Thresh is one of those champs that you need to have someone know how to work with. How many times do they not take her lamp?
And the thing about not taking the lantern is that it's built into his power budget. Take support champs who's strengths don't rely on other people paying attention.
"How can I earn S- as support?"
With great, great difficulty.
Lvl 7 Thresh here. I think it has to do with KDA maybe? I usually find if I have low deaths and high assists (and try to snaggle one kill) I'd usually get an S around that range. Example would be 2/4/21 and try to pick up a couple of farm while you're at it.
When I've gotten s+,s,s- as a support it's been the games where I've been to every lane, helped with objectives (dragon/baron/towers) and placed so many wards I was always out! was constantly checking for wards to disable, and when my carries backed I'd farm the minions to prevent losing towers. I've even gotten an S after I thought I wouldn't because I died quite a few times.
