Feature request at addon page / Addon comments at addon page / private message to the author
Search the addon via the addon search box at the top right, at the description you'll find the author name and link to click for writing a pm: top left edge
"by: <addon author>"
Or click on the comments tab and write a comment there.
At best use the feature request button if visible like within AGS addon page:
It's designed for feature requests.
Or use the "bug" button if you found a bug and want to report it.
Not all addons got this enabled though.
So if it's not visible use the addon comments.