That's the thing screwed into the top of the thermostat housing right ???? (on the TS's anyway). If it's the same as the TS engines, then you just screw it down. You won't loose coolant as it is on the top, just do it when cold though.
I replaced mine about a year ago, no leaks.
Hope this helps
woops, took the enigine temp sender unit out last night to find hot coolaent flying everywhere... quick change over meant not to much lost... so for 16V owners you need to top up after changing this sender unit
I used an old credit card to block the hole until ready to do the quick change over
Read a warning recently that cats/dogs and other local wildlife are attracted to the sweet smeel of coolant and its highly toxic......please make sure its diluted/cleaned if spilt..........wish I'd know this a few months ago while spilling mine
if it's the 16V you're talking about: the sender (with a blue bosch plug) has a solid copper washer to seal it. Use a new one, or if you don't have one and the shops are closed, heat the old one for a few moments until it glows red. This will resoften it sufficiently to seal when you put it back on.