"加苦味于" can be translated into English as "to add bitterness to" or "to impart bitterness to".
For example:
He added some bitter herbs to impart bitterness to the soup.（他加了一些苦味的草药，给汤增添了苦味。）
The chef decided to add bitterness to the dish to balance the flavors.（厨师决定给这道菜加点苦味，以平衡各种味道。）