The Item create service uses the phrase 'xa_sc_item_thread_create' to set the text for the first post. You can edit that phrase to change the wording to what ever you prefer.
NOTE: Changing the Phrase is NOT going to change the 1st post in any existing threads, the phrase is only used during the Thread Create process to set the text of the post.
This is the same exact thing that the Core XF Resource Manager does (it uses a phrase to set the text for the first post of the associated discussion thread that is created during the creation of a Resource).
There are some available variables that are passed into the phrase, that you can use to customize the phrase.
PHP:
$phrase = \XF::phrase('xa_sc_item_thread_create', [
'title' => $item->title_,
'term' => $category->content_term ?: \XF::phrase('xa_sc_item'),
'term_lower' => $category->content_term ? strtolower($category->content_term) : strtolower(\XF::phrase('xa_sc_item')),
'username' => $item->User ? $item->User->username : $item->username,
'snippet' => $snippet,
'item_link' => $this->app->router('public')->buildLink('canonical:showcase', $this->item)
]);
The default phrase only uses 'term', 'item_link' and 'title', however, you can add things like snippet if you want to (like XFRM does).
HTML:
This thread is for the general discussion of the {term} [url={item_link}][plain]{title}[/plain][/url]. Please add to the discussion here.