I don't believe you can get that number from the console of the browser. Visid _High and _low are backend ids, they are not disclosed to the front end....
You can get the Marketing Cloud Visitor ID (mid/ecid/etc):
var mcid = visitor.getMarketingCloudVisitorID(useMarketingCloudID) //useMarketingCloudID is your organization id
or if you just need to track it to a prop or eVar you can use:
s.prop1 = "D=mid";
You can also get AID or Fallback IDs if they exist... but none of those will correspond to visid_high/low
Short answer: you can't.
All "post_..." dimensions are only available in AA's backend, because they are "post processed", i.e. processed after data has been collected. You can see these dimensions in Data Feeds only.