Conditional for embed variable

I have this-cat-id variable used in an embedded template, but I can’t seem to figure how to use a conditional with it.

The variable: -embed=”group/temp” this-cat-id=”{categories limit="1"}{category_id}{/categories}”-

If the category_id == “” How do I assign a value, like 68 for example. I’ve tried many different ways where I assign the variable and where the embedded variable is embedded.