Unfortunately Dorico does not yet support this kind of relative metronome mark, sometimes (perhaps more often) called a metric modulation, though of course it is on our wish list for the future. In the meantime, you can create it using Shift+X text (or, if you want it to appear on every staff, use Shift+Alt+X system text). You can copy and paste the appropriate notes from here. Before you paste into the text editor in Dorico, make sure you change the character style to ‘Music text’ using the drop-down in the top right-hand corner of the editing popover.