I would like to add that I was able to successfully add this custom .dll in another Windows Application project and was able to add this custom .dll as reference. Thereafter creating an instance of this imported .dll I was able to use the public functions.
But somehow in my OpenSpan project when I add reference of this .dll and try to add in toolbox it doesn't show up. Nor does it gave any error
In visual studio->new project->class library. Once project loaded, delete the default class1. Now, right click on project and click on add->component. Write required methods and build the solution. Follow below instructions to use in automation.