Workflow Script Editor - adding assemblies

I like to amend a workflow (Produmex -> Organizational Structure -> Workflows) with some custom logic. For this, I need to add some .Net assemblies like System.Text.Json. If I add "using System.Text.Json" to the script and use validate, I get an error indicating the assemble cannot be found. Where do I need to put the System.Text.Json.dll to be picked up? Tried "C:\Program Files\SAP\SAP Business One\AddOns\PMX\Produmex Add-On Loader 21\<my database>", but that didn't work. Or must I run something else?

Thanks in advance for any help.