You can add custom html in payment html file at
app/code/Test/Testpayment/view/frontend/web/template/payment/mypayment.html
You can add your custom html before <div class="actions-toolbar">
<div class="payment-method" data-bind="css: {'_active': (getCode() == isChecked())}">
<div class="payment-method-title field choice">
<input type="radio"
name="payment[method]"
class="radio"
data-bind="attr: {'id': getCode()}, value: getCode(), checked: isChecked, click: selectPaymentMethod, visible: isRadioButtonVisible()"/>
<label data-bind="attr: {'for': getCode()}" class="label"><span data-bind="text: getTitle()"></span></label>
</div>
<div>ADD YOUR HTML HERE</div>
<div class="payment-method-content">
<div class="actions-toolbar">
<div class="primary">
<button class="action primary checkout"
type="submit"
data-bind="
click: placeOrder,
attr: {title: $t('Place Order')},
css: {disabled: !isPlaceOrderActionAllowed()},
enable: (getCode() == isChecked())
"
disabled>
<span data-bind="i18n: 'Place Order'"></span>
</button>
</div>
</div>
</div>
</div>
Now remove static payment template file from pub/static/frontend/vendor/theme/en_US/Vendor_Module/template/payment
Flush cache and try....!
OUTPUT: