$eavSetup->addAttribute(
            MagentoCatalogModelProduct::ENTITY,
            'custom_dropdown',
            [
             'type'                    => 'varchar',
             'backend'                 => '',
             'frontend'                => '',
             'label'                   => 'Course Location',
             'input'                   => 'select',
             'class'                   => '',
             'source'                  => '',
             'global'                  => MagentoEavModelEntityAttributeScopedAttributeInterface::SCOPE_GLOBAL,
             'visible'                 => true,
             'required'                => true,
             'user_defined'            => true,
             'default'                 => '',
             'searchable'              => false,
             'filterable'              => false,
             'comparable'              => false,
             'visible_on_front'        => true,
             'used_in_product_listing' => true,
             'is_used_in_grid'         => true,
             'unique'                  => false,
             'apply_to'                => '',
            ]
        );