I have a DLL that unity accesses, how can I load package manager text mesh pro into a external DLLL

Hi,
I upgraded our Unity project from 2018 to 2021. It complained about textmesh pro using a package manager now.
We have an externall dll (vs 2017 project) which references the old text mesh pro - How Do I tell the external dll vs project to use the package manager version of the text mesh pro dll? It doesn't use unity so there is no package manager as far as I can tell.

Also, will need to update the other unityengine.dll etc to 2021 dll's.