Buy ELSS through COIN

I would like to buy ELSS through COIN. Do I need to deposit money first or it wl get debited directly from bank?

BANK to TRADING Account First. And then to COIN.

Yes, you have to transfer money first from bank account. You can go to coin and then click add funds. In case you already hold funds in trading account, same gets synced bydefault in coin too.
Refer link.