Yes it is possible to get pinworms from a toilet seat.
The way you would get pinworms from a toilet seat is you sit down on the toilet seat and then you wipe your butt with the toilet paper and you then get the pinworms transferred to your hands.
Then if you do not wash your hands after wiping your butt and sitting on a toilet seat then when you eat or touch your mouth then you can get the pinworms.
Pinworms get into your body through your mouth by ingesting them.
So as long as you wash your hands often enough then you should not get any pinworms.
But if you do have those toilet seat covers available it's best to use them on public toilets before you sit down on them.
Or put some toilet paper on the toilet seat.
But also always wash your hands after using the restroom even when at home.