Gama11 answer is correct but -net-lib and externs should be added since they do allow you to use c#/mono code in your Haxe c#/mono project.
Haxe can consume compiled code for all targets using externs ( compiler definitions ) for use within your Haxe project for that same target, for most targets you need to create suitable externs manually. However for some targets ( Java, C#, Flash ) Haxe can auto generate the externs required, although not always successful it can make it fairly painless to use existing libraries. I have found this very successful with flash and had varied success with Java target but it is always improving, and perhaps tried it once or twice with mono ( c# target ). In your hxml complier code you may write.
-swf-lib my.swf
-java-lib my.jar
-net-lib my.dll
You cannot use a a swf or jar file with a c# project, nor a dll with a flash or java project, atleast not this way, although you can use a swf with openfl and nme when targetting c++ via special parsing libaries.
Technically for C# this will be described in the future Haxe manual more formally than I have, see link to github markdown page for full details:
"Haxe can directly load .NET assembly files (.DLL) and convert its type definitions for use as Haxe types. To load a .NET assembly, use -net-lib library.dll compiler option. Haxe will then automatically parse types defined in that assembly file and make them available for import as Haxe types." - Quote https://github.com/HaxeFoundation/HaxeManual/wiki/Haxe-C%23