如何在ElMessageBox标题前添加图标

<template>
  <div>
    <el-button @click="showMessageBox">显示消息框</el-button>
  </div>
</template>
<script>
export default {
  methods: {
    showMessageBox() {
      this.$alert('这是一条消息', '提示', {
        customClass: 'message-box-with-icon'
      });
    }
  }
}
</script>
<style>
.message-box-with-icon .el-message-box__header:before {
  content: "图标";
  display: inline-block;
  margin-right: 10px;
}
</style>