How to format 'Product Review' text box font ?

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If you put the input textbox tag directly in the threadfields_inputrow template like that, other input type won't be displayed (textarea, listbox and etc). And, XThreads input type has a "dynamic" name, something like xthreads_key, depends on the key of the custom fields, not subject.
XThreads input fields defined in the {$inputfield} variable in the template.
What's wrong using the .myinput .textbox class? It doesn't work in IE browser?

For the Template Prefix, you can find it in edit/create forum page, not in edit/create custom fields:
AdminCP -> Forums & Posts -> Forum Management -> Edit your PR forum, scroll down the page. You'll find the Template Prefix setting there.

For the reply function, you can set the reply permission for your PR forum.
And, optionally (if you don't have it yet), you can create a new showthread template, copy paste your default showthread template to the new showthread template, and name it as prefix_showthread template (again, depends on the Template Prefix used for your PR forum. Then, style it as you want (maybe you want to remove the {$newreply} variable from the prefix_showthread template.