Get customer id from JavaScript for use in secondary API call

[jbcm]

Im developing a Shopify app that needs to update a customers metafield with a value.

I have added a custom JS file to all pages using the ScriptTag API, using the app proxy I get a callback to my server, where I want to update the customer metafield using:

POST /admin/api/2022-07/customers/$CUSTOMER-ID/metafields.json

The problem is I cant find the customer ID from the JavaScript file. 

I tried this codeblock:

 var getCustomerId = function() {
    try {
      let curr = window.ShopifyAnalytics.meta.page.customerId;
      if (curr !== undefined && curr !== null && curr !== "") {
        return curr;
      }
    } catch(e) { }
    try {
      let curr = window.meta.page.customerId;
      if (curr !== undefined && curr !== null && curr !== "") {
        return curr;
      }
    } catch (e) { }    
    try {
      let curr = _st.cid;
      if (curr !== undefined && curr !== null && curr !== "") {
        return curr;
      }
    } catch (e) { }
    try {
      let curr = ShopifyAnalytics.lib.user().traits().uniqToken;
      if (curr !== undefined && curr !== null && curr !== "") {
        return curr;
      }
    } catch (e) { }
    return null;
  }

And that gives me a customer id, but its a UUID, not a numeric ID, and thus I cannot feed it into the API call to update the metafield.

I also tried adding a liquid tag, so that I could extract it from JS

<div id="app-customer-id">{{ customer.id }}</div> 

but no matter what I try and where in the shop I browse, that DIV is always blank. Im sure Im logged in as a valid customer, but on all pages that value is blank. My shop is in development-mode and is password protected.

Any help would be very much appreciated! Thanks.