How to create comment box?

That ‘Workflow’ view for actioning task is an OOTB System view that can’t/shouldn’t be modified.

You can create your own view for actioning task and add it to your form as per:

https://help.nintex.com/en-US/k2five/userguide/current/Content/How-Tos/ManualTaskIntegration/ManualTaskIntegration.htm

You can use the Workflow Comment SmartObject or use your own SmartObject to create views for adding comments to workflow as per:

https://help.nintex.com/en-US/k2five/userguide/current/Content/How-Tos/WorkflowComments/WorkflowComments.htm