To follow-up on @████████'s response:

Postman uses an external library called "RestSharp". Adding the RestSharp library reference to a Workflow Script activity requires resolving its several dependencies by adding those references as well. The following are instructions for the RestSharp version 108.0.1 .NET Standard 2.0 build in Workflow 11 Update 1. They should also serve as a general example for .NET Standard 2.0 library dependency resolution in Workflow Script activities. 

Whenever I reference a nuget package below, it means that you should download the package directly from nuget.org, use 7zip to extract it, and copy the .NET Standard 2.0 dll to your .\Program Files\Laserfiche\Workflow installation folder. Example of extracted nuget package path with netstandard2.0 dll (pre-copy to Program Files):

Drilling down through extracted nuget package (screenshot shows a different one):

Summary of steps:

1. Download RestSharp from nuget
   1. RestSharp for .NETStandard 2.0 has a dependency on System.Text.Json 5.x
2. Download System.Text.Json 5.02 from nuget
   1. The latest version of System.Text.Json is 6.x. Don’t get 6.x. RestSharp 108.0.1 (latest as of today) strictly requires 5.x and will fail if you have 6.x loaded instead.
3. Download every .NETStandard 2.0 dependency System.Text.Json lists on nuget, except for:
   1. System.Runtime.CompilerServices.Unsafe, which doesn’t seem to be needed
   2. System.Numerics.Vectors, which seems to already exist on Windows Server 2019. If you don’t find it in the GAC, download it from nuget.
      
4. In your Workflow script, add references in order:
   (If you don’t add them in this order, you might need to remove and re-add various references to resolve their own dependency warnings)
   1. netstandard version 2.0.0.0 (from GAC)
      
   2. System.Net.Http version 4.0.0.0 (should be in assemblies browser list already)
      
   3. System.Numerics.Vectors (from GAC or WF install folder)
   4. System.Buffers (from WF install folder)
   5. System.Memory (from WF install folder)
   6. System.Text.Encodings.Web (from WF install folder)
   7. System.Threading.Tasks.Extensions (from WF install folder)
   8. Microsoft.Bcl.AsyncInterfaces (from WF install folder)
   9. RestSharp (from WF install folder)
5. Add “using RestSharp;” to the “using” statement list at the beginning of the script code. You shouldn’t need to add Using statements for any of the other references.
6. Add this single line of code to the Execute() block of the Workflow script, which per https://restsharp.dev/v107/#restsharp-v107 is “how you can instantiate the client using the simplest possible way”:
   1. var client = new RestClient("https://api.example.com");
7. Add this after for a simple confirmation the script doesn’t hang on the line above:
   1. MsgBox(“Success!”);
8. Build and run the script to verify there aren’t any compile or runtime errors.
9. Success!