Posting content written by somebody else without providing a source is still plagiarism, regardless of whether they gave you permission to do so.
Having permission of author or not, leaving source link, full quote, or quote with author's name is the only way to avoid plagiarism. If someone don't have shady purposes, they don't mind to leave source link or quote.
By the way, I have only one curious question that: What happen if I the quote button to quote your post, for example, but during my time to compose my post to answer you I mis-delete the end bracket? Is this considered as plagiarism if someone else use bot to find plagiarised posts and report such posts?