https://devs.keenthemes.com/question/custom-validator-or-remote-validator-with-ajax-request

steps.js

const existClient = function () {
  return {
    validate: function (input) {
      return new Promise((resolve, reject) => {
        const value = input.value;
        if (value === "") {
          resolve({ valid: true });
        } else if (value.length > 0) {
          $.ajax({
            url: "/clientes/findClient",
            type: "GET",
            data: { "id": value },
            success: function (data) {
              resolve({ valid: data.length === 0 });
            },
            error: function () {
              resolve({ valid: true }); // or reject based on how you want to handle errors
            }
          });
        } else {
          resolve({ valid: true });
        }
      });
    },
  };
};

// Register the validator as usual
validator.registerValidator('existClient', existClient);