A "drip tip" is just a mouth piece. In earlier days of vaping, we used what's called an "atomizer", which is a cylinder with a coil at the bottom. You dripped a few drops of e-liquid directly onto the heating coil, placed a drip tip on the top and vaped. You could also vape it without a drip tip by putting your mouth on the atomizer but ran the risk of burning your lips after a while.
You can also direct drip into an RDA, a rebuildable dripping atomizer. These, you have to build your own coil for (you can now buy premade coils to put into an RDA), you put the wick into the coil, saturate with e-liquid and vape.
Your iSub tank is just that: a tank. It's not an RDA and you can't drip with it.
My advice to you is to learn. Read lots of threads on this forum, watch Youtube videos about coil building, ask lots of questions like the one you have here... and then listen to the replies and learn from them.
If you put a few drops down the tube? All you're going to get is gurgling when you take a hit, and possibly some liquid that will seep onto your center pin of your battery that will need to be cleaned off and dried. Don't do it.