Does depend on model and age. most of the ideal range now have internal filling keys which can be turned and water introduced into the primary circuit. The older models require a filling loop on nearby pipework between the primary return heating pipe and the mains cold water. only pressurize to 1 bar (while boiler is cold), and do not use while pressure is zero as you run the risk of damaging the pressurisation unit and getting lockout faults. if in any doubt call a registered CORGI plumber.