Look in code view at the sample page ;)
Right after the closing form element, there is php code that you can use for accomplishing this.
When you bind to a form element in the DataAssist insert or update record behavior, it adds code that looks like:
<?php echo ((isset($_POST["category"]))?$_POST["category"]:""); ?>
this is a ternary expression that says:
does the category form element have a value?
Yes: use the category form element a value.
No: use a blank string
The code I have added at the bottom of the page takes that a step further:
<?php echo((isset($_POST["category"])&&$_POST["category"]!="")?$_POST["category"]:$_POST["other"]) ?>
this basically says:
does the category form element have a value that is not a blank string?
Yes: use the category form element a value.
No: use the other form element a value.
you can use this code for the category column in the Insert or Update record behavior.