Subscription REST call fails as admin

I'm creating a custom component for a subscribe button. The button needs to exist on a QandAQuestionPage and subscribe users to topic changes (side note: still unclear on thread vs message subscriptions - there seems to be no documentation on the difference). The button needs to do the following on page load:

• For anonymous users, add disabled class to button
• For logged in users, check if a subscription already exists
  • If so, assign custom text to the button i.e. "unsubscribe" and assign click event to unsubscribe from thread
  • If not subscribed, button will have the text "subscribe" and click event will subscribe the user

I run into problems when I try and add a subscription. Here is the rough code:

<#assign message = page.context.thread.topicMessage! />
<#assign topicId = message.uniqueId! />

<#assign subscription = rest("/threads/id/${topicId!}/subscriptions/users/self/type/email").subscription[0]!/>
<#assign userIsAnonymous = user.anonymous! />

<#assign buttonStateClass = "button-default" />
<#assign buttonText = text.format("label.subscribe_to_label") />
<#assign buttonSubscribeAction = "add" />

<#--Check user logged in state and subscription status-->
<#if userIsAnonymous!>
  <#assign buttonStateClass = "button-disabled" />
<#else>
  <#if !subscription.@null??>
    <#assign buttonStateClass = "button-active" />
    <#assign buttonText = text.format("label.custom.subscribed_to_label") />
    <#assign buttonSubscribeAction = "remove" />
  </#if>
</#if>

<button id="watch-topic-button" class="${buttonStateClass!}" data-action="${buttonSubscribeAction!}">${buttonText!}</button>

<@liaAddScript>
  ;(function($) {
    $(function() {

      $("#watch-topic-button").on("click", function () {
        var action = $(this).data("action");
        post(action);
      });

      function post(action) {
        $.post("/restapi/vc/threads/id/"+${topicId}+"/subscriptions/users/self/"+action+"?subscription.type=email",
        function(data){
          console.log("success?: ", action, data);
        });
      }

    });
  })(LITHIUM.jQuery);
</@liaAddScript>

Checking the subscription is successful, but adding a subscription results in a Error.. in the XML response is an error code 303. When I try to hit the URL directly I get a 504 error "Method 'get' is not supported off of node 'thread_subscription_context.users.self.add'"

I was looking at code from discussions like:

https://community.lithium.com/t5/Developers-Discussion/I-need-a-way-to-add-a-subription-to-a-board-for-many-users-at/m-p/171651#M6837

I'm an admin user and copied this code from discussions here.

Any ideas? Is there a better way to do this?

Thanks