I have the syringe and tubing. Where can I get the screw part that goes over the presta valve with the core removed? Would presta to schrader adapter/converter work?

You don’t need anything to screw on – just inject it into the valve when the core is removed. Go slowly (obviously). I still have one of the small Stans bottles that I refill and use.

I just use a syringe, pull the plunger, and pour it in like a tiny funnel.

Or you could just put the tubing over the valve.

Small syringes fit into the hole. Big ones you can whittle the end down so they fit. No need for anything else. The sealant doesn’t go in under pressure so there’s no need for a particularly good fit even.

You can even do without the syringe altogether. With the valve at the top of the wheel the big bottle fits between the spokes and the hole in the lid fits neatly over the valve stem. Spin it round, give it a squirt, done. You have to guess at how much has gone in, mind 🙂