Hi,
According to you description, my understanding is that you want to fill the fields in the custom content type before this task used by assigned user.
Per my knowledge, there is no easy way to set value for the content type contains in the single task workflow.
As a workaround, you can use some workflow variable to store the value in the content type in the workflow so before assgined to user, you can do some logic business with the variables.
Here is a detailed article for your reference:
How to: Create Custom SharePoint Server 2013 Workflow Forms with Visual Studio 2012
Thanks
Best Regards