In case people haven't found it, here is the page you need, to self-book your (first) jab
At the top, there's a revolving set of slides, one of which says "Vacina Covid-19 Pedido de agendamento"
As of Friday 4 June, this is available to all Portuguese residents over 45 with an SNS número de utente and a mobile phone number. The system is easy to use, provided you have that number to hand.
You can pick a location and a day. Within a couple of days, you'll receive a text with a time slot, and you must respond to that text message to confirm or reject (otherwise it'll automatically cancel). Instructions are in the text.
The age bracket is due to be widened again later in June, with the aim remaining to give all adults at least one jab by the end of the summer.