I have a workflow built on NINTEX 2013 and need to build custom .Net form that integrates with it. How can I do this activity?
What do you need to integrate?
You could integrate using the Nintex Workflow Web Service 's methods
Take a look at >>> Methods List of Nintex Workflow Web Service
The workflow usually run on List or Site level so the form shouldn't matter as long as the item is created or updated in the list. If you need to access certain workflows from the form, I agree with Fernando Hennequin that you will have to look at the web services in Nintex.