Vaillant TCW sine T3W

I don't know this particular boiler, but most combis have a filling loop (braided flexible hose) from the mains water feed that connects with a lumpy nut to a threaded filling valve - attach this, and open the taps at both ends of the filling loop, the gauge pressure should start climbing, once it's where you want it, close the valves and remove the filling loop at one end, catching the drops in your hat.

This boiler (around 20 years + ) uses an external filling loop. See the sealed system FAQ. for details. I wouldn't worry about the pressure if it's only just under 1.