If you want to cite a work which is referenced in another work, you should try and track down the original.
However, if this isn’t possible, make it clear in your text where you found the information and only include a reference to the document you’ve read.
Dunn (2008), as cited by Campbell and Muncer (2018), believed …
or Dunn (2008) revealed that … (cited in Campbell and Muncer, 2018)
or … (Dunn, 2008, cited in Campbell and Muncer, 2018).
Your reference list will include a reference to Campbell and Muncer’s work, but not to Dunn’s.