Write a string of text to all HTML files

Hello

Can anyone help me with some VB Code to write automatically on file open a string of text which is appended to all files that end in file extension ".HTML", if these file extensions exist?

These files should be in a directory called "Z:\HTML"

I know how to write to a file but I haven’t figure out how to write to all files that end in a specific file extension so any assistance will be appreciated

Thanks
Tom