To pre poplulate text in Newsletter Block when users url like this: /newsletter?fname=Nick&lname=Hutton&[email protected]

You can use this code to Code Injection > Footer.
<script>
document.addEventListener('DOMContentLoaded', function() {
const urlParams = new URLSearchParams(window.location.search);
const fname = urlParams.get('fname');
const lname = urlParams.get('lname');
const email = urlParams.get('email');
const firstNameField = document.querySelector('input[name="fname"]');
const lastNameField = document.querySelector('input[name="lname"]');
const emailField = document.querySelector('input[name="email"]');
if (fname && firstNameField) {
firstNameField.value = fname;
}
if (lname && lastNameField) {
lastNameField.value = lname;
}
if (email && emailField) {
emailField.value = email;
}
});
</script>
