You *can* get the balance using the following form, but the lifespan of the token may be a problem: 1. Log into arcgisonline 2. Open your browser's development console, then refresh the page and watch your network tab. 3. Find the REST call tied to your credit lookup. It should take the form: http://<your_orgainization>.maps.arcgis.com/sharing/rest/portals/self?culture=en&f=json&token=<your_token> 4. Reuse this call where you intend to do the actual credit check, and use the "availableCredits" key in the JSON response. The tokens seem to expire quickly, so you may need to find a way to generate a longer lasting token.
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