Thingamajig as a noun is something that one does not know the name of. while Whatchamacallit as a noun is a metasyntactic term used for any object whose actual name you don't know or can't remember. there are many spellings and many synonyms.
Part of speech: noun
Definition: Something that one does not know the name of.
Part of speech: noun
Definition: a metasyntactic term used for any object whose actual name you don't know or can't remember. There are many spellings and many synonyms.
We hope you now know whether to use Thingamajig or Whatchamacallit in your sentence.