It was really crude.
I've done it two ways. One was by using a copper sheet sandwiched in a 1/4" pipe connector and a lock nut, to make a 'gicleur' that goes into the group head inlet port (originally meant for a dipper tube but my lever doesn't use the dipper port). The copper sheet was easy to work with because I can cut to size and punch a hole instead of drilling. No workshop access here. But to change that I have to remove the entire group head each time.
Second method : My lever group has a non-adjustable springless ball valve. So I've added some silicone sheet spacer above the stainless ball to restrict the ball movement (thus restricting the flow). In either case, it's really trial and error to find the flow rate that works.
Mine is a HX-lever that uses the water line (about 2.5-2.7 bar). But in both methods I don't find the preinfusion pressure being reduced, as long as the chamber is filled with water.