In child theme, add CPT to custom taxonomy registered in parent theme

Ok, so after another hours of digging through the codex, I found a solution. Hope this helps anyone else who is runs into the same issue. I had to use the register_taxonomy_for_object_type function. And then I used the init hook. Here is the function that worked for me.

function add_cpt_to_project_tax(){
    register_taxonomy_for_object_type('project_category', 'external_project');
    register_taxonomy_for_object_type('project_tag', 'external_project');
}
add_action('init','add_cpt_to_project_tax');