Auto populate field with custom field

Is there a way, by using the "Allow field to be populated dynamically", to have a form field automatically get populated by a custom field?

For example In my post I have a custom field called "stock_num" so in my form I would like this to be automatically populated so that it is included when the form is submitted.

I know I can set the Paramater Name of the field to "stock_num" and then use this function

add_filter("gform_field_stock_num", "populate_stock");
function populate_stock($value){
   return "";
}

But I am not sure how to pull in the custom field. Any direction would be great