New Post: Add new field

I'm not sure what you mean by a new textbox on the product detail page.. are you talking about a new tab with text content?

If that's what you mean, then there is already some example code inside the Back Office and the product details template for the purpose.

Look for the specificationstab.flag setting in the Back Office and read the help text on the page. It has the required information.
Regards
ROb