How to store additional customer data in customer notes?

[Alocard]

Hey guys,

I want to collect additional customer data and store it in customer notes.

I have found this article in the ShopifyDevs. Unfortunately this article only describes how to store additional data when creating a fresh customer account. My intention is to display a form on the customer's account page, that they can fill out at any time. The form is supposed to look something like this:

<form>
<label for="custom-id">Custom ID: </label>
<input type="number" name="custom-id" id="custom-id" />
<input type="submit" value="Save ID" />
</form>

I probably have to build a small app to accomplish this. So i read the ShopifyDevs and tried building an app. My approach was to create a ScriptTag that renders the above form to the customer's page and store the input to the right customer note. That didn't work, because i couldn't use the GraphQL API in my ScriptTag.
I am looking for a hint in the right direction. What is the recommended approach? 
I am looking forward for your answers!
Thank you in advance!

[BaileyPaserk]

Have you gotten anywhere with this?