Teleporting called players between dimensions/locations

Right so, im creating a mod specifically designed to mess with someone ill be playing with, all relient on commands and stuff only I can execute.  One of these commands, teleporting a specific player of my choosing to where i am, both in location in dimention, is frustrating...because i dont know how to set *them* as the target entity.  once i get how to do that, id be completely set for that and a few other sneaky commands, but im completely stuck until then.  

the way i have it rn is i put their username into a command parameter, but then its stuck as that type.  Any ideas?

(should mention btw im using the 1.12.2 plugin, due to that being the version of the modpack we're playing)