The booster is meant only to pre-inflate your tire so it sits on the rim tightly enough to keep the air in. You then have enough time to insert the sealant through the milKit valve, using the milKit syringe and needle, screw the valve core back and to inflate the tire to full pressure until it fully jumps onto the rim.
If your tire doesn’t hold the air well after pre-inflation with the booster, you might have to first screw the valve core in, inflate the tire to full pressure until it fully jumps onto the rim, then reduce the tire pressure to below 1.5 bar / 22 psi to add the sealant into the tire.