Disable/Enable a button on EC screen

Hi,

I need to enable the Create new button on custom EC screen embedded into standard opportunity

only when the Competitor info is maintained in opportunity. Is there a way to do this?Any suggestions are helpful.

I see the option Calculation rule in the Advanced option of Enable property for Button.But not sure if I can write code for Opportunity

Query execution and then to access competitor info from there.The syntax is showing some Ruby error.

Regards

Jayasri