You can add tannin both before and/or after.
Adding tannin in cider is similar to adding tannin in wine.
There are two types of tannins and both have their place ... one is the type of tannin from grapes and the other type is from wood - usually oak.
The tannin powder most people get from the brewers/winemakers store is generally the *grape* type unless specified otherwise.
Grape tannin is typically added before ferment (particularly with wine) ... Oak tannins typically after ... although if all you have is the grape type, you could use that after as well to carefully adjust to taste.
Grape tannin in particular adds a more bitter element as it is derived from catechins in the stems and seeds of grapes.
This is why if you are *not* getting the tannic bitterness from the apples, and decide to add grape tannin powder, you should be careful with the additions. Grape tannin can make your wine or cider bitter quickly if overdone.
You should also be careful if you add oak tannin too, even though the “hydrolyzable” type of tannins that oak adds are softer and add more favorable flavors and aromas without the tendency to quite as much bitterness.
While the bitterness of both types of tannin will mellow over time, the corresponding rise in astringency in that molecular change as it ages will seem greater with the grape tannin because it already has added more astringency to begin with. (astringency is the perception of dryness in the taste of wine or cider - or if really too astringent, "roughness")
For taste, having an adequate amount of tannin can balance and improve the brightness of the cider without necessarily having to add additional acid, as tannin tends to slightly reinforce the taste of acidity. Both acidity and bitterness (and astringency to some degree) are what avoids a cider being insipid or “flabby”.
Tannins also help protect against oxidization and the oxidized browning of cider by scavenging oxygen ... especially after fermentation. A good thing if you expect to hold or age the cider at all.
Personally, when I add tannin after fermentation I add the type of tannin from oak ... either oak cubes, chips or powder but I like cubes the best.
You still have to be careful when doing this ... err on the side of too little rather than too much. A half ounce medium toast french oak (or American) cubes for 3 to 5 gallons in the secondary and let sit for 2 to 4 weeks, tasting periodically to gauge the speed that the oak is taking effect. Then, rack off the oak when it is to your liking.
Importantly, not every addition works the same and keeping track by taste is important.
Oak cubes give out their effect more slowly than chips and so you can control them more easily. I’d suggest avoiding oaking *powder* entirely ... but the powder is available too and works ok if you’re careful.
If it’s a big batch of cider and you haven’t oaked cider before, consider making a part of that batch with *no* additional tannin or oaking at all, so that if you have to blend to get the flavor right at the end (that is, blend out some of the effects of too much tannin), you will have part of the batch to do that with.
This is a good idea too if you are adding grape tannin to a large batch that you don’t want to have to correct by fining out the tannin later as well.
Personally, I use grape tannin before the ferment ... and the oak tannin from “oaking” after primary ferment.
You can use both ... but me mindful of the cumulative amount of tannin ... and that if you end up with too much you might need to fine-out (with fining agents) or blend-out excess tannin with cider that has not had it added.