Do you have to add the Apple Watch to the plan if you buy it from a third party

Firstly who are the third party? An Authorised Apple dealer? Second what plan? Third why not buy from Apple? Buy third party you don’t get the same protection as buying from an Apple or Authorised dealer. You can fall in to so many pitfalls. Is the Watch new or secondhand?

Make sure the third party, if it is a used Apple Watch, follows this procedure

https://support.apple.com/en-us/HT208855

if the Apple Watch is a Cellular model, you do not need to activate the cellular capabilities, and you can use is in a GPS only mode.

If it is a GPS only model, then there is nothing to add to any plan.

If it is a Cellular model AND you want to use the cellular capabilities, you have to add it to you iPhone cellular carrier (if and ONLY if you carrier supports the Apple Watch), and your carrier will add a $5 to $10 a month additional to you bill.