To use "MouseUp" in REALbasic, use "MouseDown"

You may want to detect the user's clicks on controls other than buttons in your REAL Studio 2010 window. For example, you may display an advertisement in a Canvas object and want to ShowURL when the user clicks the ad.

To respond to the user's click, you write some code in the ‘MouseUp’ event handler. For example, the call to ‘ShowURL’ for the ad would go there.

There's one catch…
To enable that ‘MouseUp’ event handler, you must also put code in the ‘MouseDown’ event handler. This one line will do:

Return True // to enable MouseUp