Select all for subform field

Select all for subform field

I have a SCHEDULE form with a SITE dropdown and an ALL SITES check box.
When the user clicks the ALL SITES checkbox, I use the code below and it works perfectly

if (input.ALL_SITES)
{
    SITE_ID.selectall();
}

I have an AGREEMENT form that has a SCHEDULE subform. To replicate the same logic I have tried the following code which I generated via the script builder

row.SITE_ID.selectall(); 

But I get the following error message. 
'row' does not exist in this form

How can I use the select all function in my script when referencing subform variables?