Since .NET 5.0, dotnet fsi ships with dependency manager plugins support that can be called like so:

#r "myextension: my extension parameters"

The initial RFC for this feature overviews how it is designed.

The current implementation follows pattern that can be deducted by refering to implementation in DependencyProvider.fs; due to the system working without having a staticically linked dependency, it uses a late binding approach leveraging runtime reflection to identify if an extension conforms to patterns understood by the specifics of the compiler implementation.

Reference nuget packages, ships by default with dotnet fsi.

#r "nuget: Newtonsoft.Json"
// Optionally, specify a version explicitly
// #r "nuget: Newtonsoft.Json,11.0.1"

open Newtonsoft.Json

let o = {| X = 2; Y = "Hello" |}

printfn "%s" (JsonConvert.SerializeObject o)

Reference dependencies (nuget, git, gist, github) through Paket package manager.

Learn how to use Paket FSI integration.

#r "paket: nuget FSharp.Data"

open FSharp.Data

type MyCsv = CsvProvider<"""
X,Y
2,Hello
4,World
""">

for r in MyCsv.GetSample().Rows do
  printfn "%i = %s" r.X r.Y