The README1 of pandoc gives the solution. You need to define a dummy nocite
metadata field and put the citations there:
# References
The bibliography will be inserted after this header. Note that
the `unnumbered` class will be added to this header, so that the
section will not be numbered.
If you want to include items in the bibliography without actually
citing them in the body text, you can define a dummy `nocite` metadata
field and put the citations there:
---
nocite: |
@item1, @item2
...
@item3
In this example, the document will contain a citation for `item3`
only, but the bibliography will contain entries for `item1`, `item2`, and
`item3`.