应募 is a formal, literary verb meaning 'to respond to a call for recruits' — historically used for military conscription or volunteer campaigns. It's a separable verb (离合词): the object 募 can split off from 应 when inserting measure words or modifiers, as in 应过募 ('once responded to a recruitment call'). You'll encounter it in historical or official contexts, not everyday conversation. The pinyin is yīng (first tone), not yìng.