I had that same syringe from Effeto Mariposa.  I removed the piece that threads on and just slid the hose over the valve stem with the core removed to inject the sealant.  The hose is a tight enough fit that there's no mess.  Just make sure you clean out the syringe after every use.  I forgot to do that once and the sealant residue dried up leaving me unable to move the plunger.  I eventually broke it trying to get it free.