This is how the core achieves this:

Javascript

file: vendor/magento/module-checkout/view/frontend/web/js/view/payment.js:98

getFormKey: function () {
            return window.checkoutConfig.formKey;
}

HTML

file: vendor/magento/module-checkout/view/frontend/web/template/payment.html:19

<form id="co-payment-form" class="form payments" novalidate="novalidate">
    <input data-bind='attr: {value: getFormKey()}' type="hidden" name="form_key"/>
            ...
</form>