Book and hold was introduced on the UA website several years ago. When it first was made available you had to be an upper tier elite (1k or 1P) to use, IIRC. As far as I know it is now available to all users who have a UA MP account. There are some exceptions, for example certain fare classes or if you're making a reservation that a 24 hour hold would put you inside the window for advance purchase.

The site is more stable than it used to be, but for those of us who formerly had the pleasure of using the United Connection software ua.com will never match up.