[Vuejs]-How can I add success message after button is clicked?

You can do something after the Promise is returned, like :

await http.post("/message", {firstName: this.firstName, lastName: this.lastName, message: this.message, numbers: this.checkedNumbers}).then((response) => {
  // Do something with the response
})

Then, you can pop a swal from SweetAlert documentation with the message you want to show.