Force category choice before creating new post

Use the ‘save_post’ hook to programmatically enteract with your new post when it is created, then use the wp_set_post_terms() function to assign your term,

add_action('save_post','set_post_default_category', 10,3);
function set_post_default_category($post_id, $post, $update){
  if($update) return; //only want to set if this is a new post!
  if('post' !== $post->post_type) return;  //only set for post_type = post!
  $term = get_term_by('slug', 'my-custom-term', 'category');//get the default term using the slug, its more portable!
  wp_set_post_terms( $post_id, $term->term_id, 'category', true );
}