Actually, I managed to create a button on custom page to subscribe users to a specific category.
Add an HTML widget and add the following piece of code in it:
<div data-view="Tooltip" class="tooltip tooltip--day forum-subscribe-button" data-mn="ItemView" data-mn-app="frontend">
<button class="btn--toggle qa-forum-subscribe-button js-tooltip-trigger tooltip-trigger js-tooltip-trigger"
data-component="subscribe"
data-data='{
"isGuest": 0,
"url": "/subscribers/subforum/3",
"strings": {
"subscribed": "Unsubscribe",
"unsubscribed": "Subscribe"
}
}'
aria-haspopup="true">
<span class="text">Subscribe</span>
<span class="loader"></span>
</button>
<div class="tooltip__content tooltip__content--subscribe js-tooltip-content position--bottom">
<i class="arrow"></i>
When you subscribe we will email you when there is a new topic in this category
</div>
</div>