Adding Options to Import page

I am trying to add options to the import page (such as 'skip duplicates') by adding a checkbox input in the page editor under HTML mode. I am then trying to get the value of the checkbox when I am importing the records.
I tried to use the following help topic to do this:

https://xlinesoft.com/asprunnernet/docs/add_custom_field_to_form.htm
However I cannot seem to get the value in my C# code. The postvalue["test"] does not exist when I am in the 'Import page: Before process" function.