When asking about 'widening a forehead', one has to know more specifics about what that means or what your objectives are. Forehead augmentation can be done through a scalp incision to build out the forehead with onlay material but it usually lies on the bone within the anterior temporal lines. Building out or widening a forehead behind the temporal lines is problematic as that requires lifting up part of the temporalis muscle, which will can easily be done but almost always results in muscle atrophy later potentially exposing the edges of the built out area if they are not perfectly smooth. This is what I would call widening of the forehead. Augmentation can be done of the temporal hollows, although that would not normally be considered part of the forehead.
Web reference: http://www.eppleyplasticsurgery.com/browlift.html