So you've got a:
<body onload="doMyOnload();">

in your freemarker template, but

in the decorator blows this away...

The site mentions
<body onload="<decorator:getProperty name="body.onload" />">
but you're not using jsp because you've convinced yourself you're better than that.

So you eventually figure out...

<body ${["body.onload"]}>

And you're all set again. Isn't that nice. 29 tabs died to bring you this information. Use it wisely.

UPDATE the commenter speaks the truth. I'd switched over at some point but never updated this.
<body onload="${["body.onload"]?default("")}">