The documentation says that you can put a DLL in a bin folder and reference it using a special #r syntax, however in the Azure portal I cannot find
bin
#r
You can add assembly reference with a relative path. In portal.azure.com -> function apps, on the right hand side, View Files -> upload the dll (eg: YourDllName.dll). In run.csx, enter #r "./YourDllName.dll"