Mail Merge Thunderbird HELP!

Is there anybody using Mail Merge in Thunderbird ?Im trying to use the Variables
What I want to do is use {{custom1}} and if there is not a custom1 use {{custom2}}
the Help off the plugin page

Variable: {{name|if|then|else}}
If the value of the field name equals if, then the variable will be replaced by then, else by else.

Variable: {{name|*|if|then|else}} (contains)
If the value of the field name contains if, then the variable will be replaced by then, else by else.

Variable: {{name|^|if|then|else}} (begins with)
If the value of the field name begins with if, then the variable will be replaced by then, else by else.

Variable: {{name|$|if|then|else}} (ends with)
If the value of the field name ends with if, then the variable will be replaced by then, else by else.

Variable: {{name|==|if|then|else}} (equal to)
If the value of the field name is equal to if, then the variable will be replaced by then, else by else.

Variable: {{name|>|if|then|else}} (greater than)
If the value of the field name is greater than if, then the variable will be replaced by then, else by else.