Hello,

1. First create a subcategories dependent values list in the module:

2. Then populate it with values in the module:

3. Then go to forms builder for the Market module and add the Subcategory field as follows:
http://yoursite.com/studio/builder_forms.php?page=fields&module=bx_market&object=bx_market&display=bx_market_entry_add

And you're done.