What is the direct URL for "watching first post" notification subscription?

I saw some topics on this, but none I saw had a simple direct URL.
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Basically, I want a direct URL that subscribes a user to a category when they click it on my site, which then notifies them by e-mail when a new topic is made in that category.

Basically, I have this code below that is a Subscribe button and when clicked, a pop-up box appears with a checkbox. If you check the box and press Subscribe, you get an e-mail when a new blog post is made. The way it works is subscribing to the category, but I am trying to find a way to convert it from the code that it is to just a direct URL. Either way, I need a direct URL for my purposes.

<% if @category_user.present? %>
  <%= render partial: 'modal', locals: {
    title: "Subscribe",
    class: "subscribe",
    content: render(partial: "subscription_form", locals: {
      category_id: @category_user.category.id,
      subscribed: @category_user.notification_level >= CategoryUser.notification_levels[:watching_first_post]
    })
  } %>

Can you elaborate on what context you would require a direct URL to understand better?

You have this default categories watching first post setting if you want to define this behavior by default. (Note: You will be asked whether you want to overwrite the user’s preferences.) Would that help?

Just something specific I’m doing on my forums. That setting won’t help, unfortunately. Just need a direct URL.

You could use the API.

Specifically POST /category/ID_HERE/notifications route with the payload notification_level: 4.

However, the context is unclear. If you could give more specific details about what you’re trying to achieve, we could give you a more precise answer.

I would like a button, that when clicked/tapped on, it subscribes you to a category basically, and sets it to “Watching First Post/” Not sure how to tie in that above POST /category/ID_HERE/notifications API stuff to when a button is pressed.

Basically, I have this code below that is a Subscribe button and when clicked, a pop-up box appears with a checkbox. If you check the box and press Subscribe, you get an e-mail when a new blog post is made. The way it works is subscribing to the category, but I am trying to find a way to convert it from the code that it is to just a direct URL. Either way, I need a direct URL for my purposes.

<% if @category_user.present? %>
  <%= render partial: 'modal', locals: {
    title: "Subscribe",
    class: "subscribe",
    content: render(partial: "subscription_form", locals: {
      category_id: @category_user.category.id,
      subscribed: @category_user.notification_level >= CategoryUser.notification_levels[:watching_first_post]
    })
  } %>