dynamic dropdown[…] doesn’t show up in the dashboard
that’s how it is designed.
Dynamic dropdowns can have multiple sources (posts, taxonomy, filter…) and therefore it is difficult to constrain its mapping to taxonomy. (it would require quite a lot of extra coding/maintenance to make sure the mapping only works for taxonomy sources.
Hence, create a normal dropdown menu with no options (or checkboxes), and map it to a taxonomy. The Post My CF7 plugin will then automatically populate it with the terms of the taxonomy you mapped it (so you get the same result as the dynamic dropdown).