JSFiddle - React, Tailwind, and code Playground
by seankoole
HTML
<div id="edit-horses-riding-items-0-item-entry-extras" class="js-webform-checkboxes webform-options-display-one-column form-checkboxes">
<div class="form-item js-form-item form-type-checkbox js-form-type-checkbox form-item-horses-riding-items-0--item--entry-extras-a3e3g0000004ugjuaa js-form-item-horses-riding-items-0--item--entry-extras-a3e3g0000004ugjuaa checkbox">
<label for="edit-horses-riding-items-0-item-entry-extras-a3e3g0000004ugjuaa" class="control-label option">
<input data-drupal-selector="edit-horses-riding-items-0-item-entry-extras-a3e3g0000004ugjuaa" class="form-checkbox" type="checkbox" id="edit-horses-riding-items-0-item-entry-extras-a3e3g0000004ugjuaa" name="horses_riding[items][0][_item_][entry_extras][a3E3G0000004ugjUAA]" value="a3E3G0000004ugjUAA">
Extra Hay (£12.00)</label>
</div>
<div class="form-item js-form-item form-type-checkbox js-form-type-checkbox form-item-horses-riding-items-0--item--entry-extras-a3e3g0000004xsiuaq js-form-item-horses-riding-items-0--item--entry-extras-a3e3g0000004xsiuaq checkbox">
<label for="edit-horses-riding-items-0-item-entry-extras-a3e3g0000004xsiuaq" class="control-label option">
<input data-drupal-selector="edit-horses-riding-items-0-item-entry-extras-a3e3g0000004xsiuaq" class="form-checkbox" type="checkbox" id="edit-horses-riding-items-0-item-entry-extras-a3e3g0000004xsiuaq" name="horses_riding[items][0][_item_][entry_extras][a3E3G0000004xsIUAQ]" value="a3E3G0000004xsIUAQ">
Start Fee (£17.00)</label>
</div>
<div class="form-item js-form-item form-type-checkbox js-form-type-checkbox form-item-horses-riding-items-0--item--entry-extras-a3e3g0000004xtbuaq js-form-item-horses-riding-items-0--item--entry-extras-a3e3g0000004xtbuaq checkbox">
<label for="edit-horses-riding-items-0-item-entry-extras-a3e3g0000004xtbuaq" class="control-label option">
<input data-drupal-selector="edit-horses-riding-items-0-item-entry-extras-a3e3g0000004xtbuaq"...
JavaScript
var $checkbox = $('label:contains("Test")').find(':input[name*="\[entry_extras\]"]');
$checkbox.attr('checked', true)
.attr('disabled', true);