short answer:
you can’t.
(well, you can – see below) as – simplistically speaking – websites cannot access hardware and make it do things.
just imagine one were to be able to write script of some sort that accesses your/any printer from a webpage or something and forces it to print 100000000 pages automatically without you being able to do anything about it.

long answer:
bearing the above in mind (again, somewhat simplified), it is – essentially – the printer that has to be able to control what does and what does not get printed automatically.

3 options spring to mind here.

a) using a cloudprint service:
your printer is registered to a cloudprint service and picks up and prints whatever is sent to that service

b) your printer is enabled to check a dedicated email account and prints whatever arrives at that account

c) your printer can listen to some script/command.
gprs printers – for example – usually have an API that one can use to let it do things
(probably to be used in conjunction with this https://www.wp-pizza.com/topic/after-every-order/)
as there is no generic/standard API that would work for all gprs printers it will be something you would have to script yourself

if anyone knows of another solution – or i am missing things – let me know and i’ll happily add/amend this here