How to test a validation function that is outside of react class using jest/enzyme

I am trying to test an function that is used to validate an empty fields in a from. This functions is kept outside of the main react class (i.e. LOGIN) and it is called using prop validations in Input form field.

In enzyme/jest test, I want to set an empty value to my Input fields to check if the function required is called atleast once.

  if (!value) {
    return (
      <div className="alert alert-danger" role="alert">
        This field is required!
      </div>
    );
  }
};

This is the function I want to test.

This is the main class just below the function where it is called in Input validations={required}

  constructor(props) {
    super(props);
    this.handleLogin = this.handleLogin.bind(this);
    this.onChangeUsername = this.onChangeUsername.bind(this);
    this.onChangePassword = this.onChangePassword.bind(this);

    this.state = {
      username: "",
      password: "",
      islogin:false,
      loading: false,
      message: ""
    };
  }

  onChangeUsername(e) {
    this.setState({
      username: e.target.value
    });
  }

  onChangePassword(e) {
    this.setState({
      password: e.target.value
    });
  }

  handleLogin(e) {
    e.preventDefault();

    this.setState({
      message: "",
      loading: true
    });

    this.form.validateAll();

    if (this.checkBtn.context._errors.length === 0) {
      AuthService.login(this.state.username, this.state.password).then(
        () => {
          this.setState({
            islogin: true
          });
          this.props.history.push("/profile");
          window.location.reload();
         
        },
        error => {
          const resMessage =
            (error.response &&
              error.response.data &&
              error.response.data.message) ||
            error.message ||
            error.toString();

          this.setState({
            loading: false,
            message: resMessage
          });
        }
      );
    } else {
      this.setState({
        loading: false
      });
    }
  }

  render() {
    return (
      <div className="col-md-12">
        <div className="card card-container">
          <img
            src="//ssl.gstatic.com/accounts/ui/avatar_2x.png"
            alt="profile-img"
            className="profile-img-card"
          />

          <Form
            onSubmit={this.handleLogin}
            ref={c => {
              this.form = c;
            }}
          >
            <div className="form-group">
              <label htmlFor="username">Username</label>
              <Input
                type="text"
                className="form-control"
                name="username"
                value={this.state.username}
                onChange={this.onChangeUsername}
                validations={[required]}
              />
            </div>

            <div className="form-group">
              <label htmlFor="password">Password</label>
              <Input
                type="password"
                className="form-control"
                name="password"
                value={this.state.password}
                onChange={this.onChangePassword}
                validations={[required]}
              />
            </div>

            <div className="form-group">
              <button
                className="btn btn-primary btn-block"
                disabled={this.state.loading}
              >
                {this.state.loading && (
                  <span className="spinner-border spinner-border-sm"></span>
                )}
                <span>Login</span>
              </button>
            </div>

            {this.state.message && (
              <div className="form-group">
                <div className="alert alert-danger" role="alert">
                  {this.state.message}
                </div>
              </div>
            )}
            <CheckButton
              style=
              ref={c => {
                this.checkBtn = c;
              }}
            />
          </Form>
        </div>
      </div>
    );
  }
}

Any help on this? I have been stuck here.