You can add your website on the "create a post" screen in the "create post from web page" field. The website URL will then show up in the caption of your post. Once shared to social media, the website will show up as clickable in the caption of your post. You can also add your website directly on the final share screen in the caption of your post.
Just so you know if you start with a link (no text or photos) in the create-a-post screen we will grab meta data from your website to pre-load the create-a-post screen with some photos and/or text to save you even more time.