How do I include an in-text citation of a source that mentions another source?

A secondary source may refer to a primary source. When citing the secondary source, only include this publication in the reference list. In the text also include the primary source:

(Siegel et al., 1995, in Wolf, 2021)

Wolf, J. (2021). Pathways to empowerment: Methodically working on participation and self-direction. Uitgeverij Coutinho.

It is recommended to find the original publication and cite only this primary source.

See also § 8.6 Primary and Secondary Sources of the Publication manual.