InputSide.js

// Omit above code for brevity

const handleSubmit = async (e) => {
  e.preventDefault();

  setButtonLoading(true);

  var dict = [];

  dict.push({
    key:   "Name",
    value: name
  });

  dict.push({
    key:   "Email",
    value: email
  });

  dict.push({
    key:   "Phone",
    value: phone
  });

  dict.push({
    key:   "Message",
    value: message
  });

  const postData = {
    contactIdentifier: '[email protected]',
    formData: dict,
    emailCampaignId: 'Your Email Campaign Item ID',
    source: "ListManager"
  };

  const response = await fetch('https://yourapi.com', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json',
      'Access-Control-Allow-Origin': '*',
    },
    body: JSON.stringify(postData),
  });

  if (response.ok) {
    navigate('/success');
    setButtonLoading(false);
  } else {
    alert('Failed to submit form');
  }
};

// Omit below code for brevity