Of course you need to start with an item link. If you don't have one I suggest looking at the most popular items.
Paste the 1688 item link into your agents search bar. The item will open natively and you can just order it (unless the seller specified that there's a minimum order quantity you're not meeting). If you ordered from Taobao or Weidian before this is the same.
But watch out: Bulk order discounts are not automatically applied by the agent. You need to open the 1688 product page to see the different thresholds to reach a bulk discount. If you qualify for a discount, you can edit the price on the agent's product page before you add the item to your cart. Don't forget!