designers chair ?

You can use designer chair or designer chairs, depending on how many of these designer chairs you're referring.

You cannot use designers chair or designers chairs because the term "designer" refers to the one designer brand, not multiple.

So if you have designer anything, just say:

Designer [Anything].

If you are talking about a specific designer (person) who made the chair, then you would say "The designer's chair".

Designer chair 是正確的。
是設計師設計的chair是只有1個人設計的，所以是Designer Chair而不是Designers Chair.