If you are referring to a Protestant belief that is not explicit in scripture but is rather passed down through teaching and word of mouth, I would say their biggest tradition is their salvation formula for “accepting Christ.” This involves saying the sinner’s prayer, which is an acknowledgment of one’s own sinfulness and the need for salvation, the acknowledgement of Christ’s payment of sins on the cross, and a formal invitation for Christ to come into that person’s heart to be his/her Lord and Savior.
It is reliably passed down through generations, and it is central to Protestant theology. No Protestant (at least in the Evangelical camp) questions this forumla, yet it comes to them through their own tradition, rather than scripture.
While this is not the normative process for becoming a Christian, I believe in ignorance of the Catholic Church, it is effective. It is a good example to illustrate to Protestants that they are not unfamiliar with the concept of tradition and it may help them become open to learning more about Sacred Tradition in the Church.