I'm not sure where you see the code. Is it a custom template?
The code in the original template looks like this:
HTML Code:
<ul class="sign-in_block">
<li class="account-link-sign_in">
{{ widget('XLite\\View\\Button\\PopupLogin', label='Sign in / sign up') }}
</li>
</ul>
So, it's the \XLite\View\Button\PopupLogin widget that renders the button and adds all the necessary parameters.
If you need a custom button, you should implement a similar widget class that will render the button with your parameters.