官网例子

这样并不是我们想要的输入框，所以可以优化一下

1、代码如下

const h = this.$createElement;
      this.$msgbox({
        title: "确认回复",
        message: h(
          "div",
          {
            attrs: {
              class: "el-textarea"
            }
          },
          [
            h("textarea", {
              attrs: {
                placeholder: "请输入回复内容",
                class: "el-textarea__inner",
                autocomplete: "off",
                rows: 15,
                id: "commentContent"
              },
              value: this.commentContent,
              on: { input: this.onCommentInputChange }
            })
          ]
        ),
        showCancelButton: true,
        confirmButtonText: "确定",
        cancelButtonText: "取消",
        beforeClose: (action, instance, done) => {
          if (action === "confirm") {
             // 点击确定 要做的事情
          } else {
            done();
          }
        }
      }).then(action => {
         this.$message({
          type: "info",
          message: "message"
        });
      });

2、要注意的是，此时vue双向绑定不生效，需要手动监听一下数值变化

 // 监听输入框值
    onCommentInputChange() {
      this.commentContent = document.getElementById("commentContent").value;
    },

3、效果如下