匿名 is a verb meaning 'to remain anonymous' or 'to keep one's name hidden'. It often appears before another verb to describe doing something anonymously (e.g., 匿名举报 'report anonymously'), or before a noun as a modifier (e.g., 匿名信 'anonymous letter'). Unlike English 'anonymous' which is an adjective, 匿名 functions as a verb in Chinese — you can say 选择匿名 'choose to be anonymous' or 匿名捐款 'donate anonymously'. It cannot take a direct object; instead, it modifies the action or the noun.