// Map the usermeta field wp_field_key to the Groundhogg field GHFieldKey

$fields_map = ​wp_fusion()->settings->get( 'contact_fields' );
​$fields_map['wp_field_key'] = array( 'crm_field' => 'GHFieldKey' );
​wp_fusion()->settings->set( 'contact_fields', $fields_map );