Your model number seems to be short a few didgits so you will have to choose one from the units is the Matching Model Numbers section.
Parts for HTS18GB - AppliancePartsPros.com

Before resorting to a drain heater tghere are a couple things to try.

First make sure that the drain tray under the evaporator coils is nice and clean.
Any dirt etc. on it can slow down the draining water giving it a chance to freeze.
Sometimes a buffed up coating of wax on it will speed up the drain water.

Check/clean the drain tube.
Sounds like drain tube is plugged so that the water from the defrost cycle cannot drain into the drain pan.

I would check that it is clear, could be that it is partially plugged.
Check that it is not plugged with dust etc. at the drain pan end.
You might as well clean the condenser/compressor fan and the condenser coils while you are down there.

If it looks like stuff is growing in the tube I would flush it with a 10% bleach solution. This should kill the stuff and let you flush it out. You may have to do this several times, let it sit for 15 minutes or so between flushes. A turkey baster comes in handy here.
Be careful that the drip tray does not overflow and empty the drip tray after you are done.

Be sure to unplug the unit while doing this.

Now for the drain heater.
I cannot tell from the picture of your defrost heater is a calrod or glass type.
I have not tried the below on a glass heater. I am a little worried that the cold spot created by the heating wire may cause the heating element to crack/shatter.

Get a piece or 12 or 14 gauge bare copper wire.
Wrap it around the defrost heater a couple times. Should be snug not tight.
Not route the other end down the drain about an inch.
Now when it defrosts it should also defrost the drain inlet.

it is a good idea to then let the unit get below freezing again and then force a defrost cycle so you can check that the wire does not get too hot.

The above is a jiggery pokery fix.
Below is a more official fix but I do not know if it would fit your unit.
Part number: AP3119957

my ge fridge comes with a simple 10g aluminum wire that goes around the heater and dips into the drain hole. You can make something up yourself. Like copper wire conducts well.