well is their any step by step procedure to implement that as I don't know How to achieve that & what files to modify?

thanks

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Did you want them to be able to actually suggest a category, or dynamically create it? If it is the first one...just create a new field called 'desired_category', and then they can enter their desired new category in there. Then when you go to validate the link, you can decide if you want to create that new category.

Hope that helps

Andy