Formula URL or Formula Text Button that will perform an action, without the need to navigate away from the current dashboard/report/record?

No there is no onload required.

I see the problem.  :)

�Item Marked Completed�, 

Those quotes in your formula are curly  66 99 type quotes that get generated typically by Word.

The need to be the ' kind of  single quote.  ie not � � and but a ' and '.