Add the following to Settings > Advanced > Code Injection > HEADER.
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
Add the following to Page Settings > Advanced > Page Header Code Injection for the contact page.
<script>
$( ( ) => {
/*
add placeholder text to first and last name fields in "Get in touch!"
form for v7.1 site.
SS version : 7.1
*/
const dataFormId = '601ced4f7050527e47ff6076';
const firstNamePlaceholderText = 'first';
const lastNamePlaceholderText = 'last';
// do not change anything below, there be the borg here
let $form = $( '[data-form-id="' + dataFormId + '"]' );
$( '.first-name input', $form )
.attr ( 'placeholder', firstNamePlaceholderText );
$( '.last-name input', $form )
.attr ( 'placeholder', lastNamePlaceholderText );
} );
</script>
Let us know how it goes.