Is this correct syntax for ES2015 tagged template?

Question

On this page there is the following code snippet (which can also be found here), demonstrating some use of a template string. The only explanation or context is the comment at the start, which makes sense to me on its own. However the snippet itself is a mystery to me still.

    // Construct an HTTP request prefix used to interpret the replacements and construction
    GET`http://example.com/bar?a=${a}&b=${b}
    Content-Type: application/json
    X-Credentials: ${credentials}
    { "foo": ${foo},
      "bar": ${bar}}`(myOnReadyStateChangeHandler);

I'm really fine with the string literal itself, it is the context that is confusing me.

Is GET a custom function, possibly loading up some fetch()? If so, is there some naming convention somewhere that is ok with full caps on a method name for GETs and POSTs? Where would you use a snippet like this? If it is to be inserted into some function called GET, why would you put it all together like this here, when the function is just going to break it into so many pieces anyway?

And finally - what's up with the parentheses at the end? This is really the bit that has me asking the question. I can't find any example elsewhere of a string literal followed by a parenthesis. What's going on with this syntax? I've spent so long searching this, that I am tempted to say it is a mistake. If this last bit was missing, the whole thing would make sense as a tag function.