Alternatively, if you are creating the theme yourself and/or can modify it, you can create an action yourself using WordPress'
do_action function. This is also how they create their other hooks. So basically in your theme, you would go where you want to, right after the
<body> tag, and do something like:
You can also pass arguments to the action callback, see the linked documentation for information.
Then afterwards, you would simply use the
add_action function to hook onto it.
Hope that helps. Sorry if I misunderstood.