He also wants it done by election day and is promising pardons to anyone who breaks the law in his name. He evidently feels not getting his wall built before then will undermine the devotion his supporters shows towards him and be an embarrassing defeat. The White House response to the fact that he's urging aides to violate due process and just steal land and promising pardons is the usual 'He's just joking'.
It sounds breathtakingly illegal and Trump will get bogged down in the courts if he actually tries this harebrained scheme.
A summary that isn't behind a paywall here.