IMHO, foam brushes are not a good choice for applying varnish. They don't wear well and can introduce bubbles in the finish. I only use them .
Their greatest selling point is they are inexpensive. But when it comes to applying a smooth coat of clear finish, such as polyurethane, a foam brush is not my first choice.. It will even start to dissolve as you are brushing on stains and finishes containing mineral spirits or lacquer thinner.
Brushes that fit into a paint can are available with either a foam or a bristle applicator. The finished look of your project will differ depending on the type of brush .
Dip the foam paintbrush into the polyurethane. Fill it only half way up. Overloading will cause drips. Begin painting the surface with long, smooth strokes.