wpmem_set_user_product()

// Basic usage of how the function is applied:
wpmem_set_user_product( $product_meta, $user_id, $date );


/*
 * The practical example below will set a default
 * membership for a user when they register. In this
 * example, it will set access for 'my_basic_product'.
 * In a real world application, get the slug for the
 * membership you want to set at registration from the
 * Membership > All Products table.
 *
 * The example uses the 'wpmem_post_register_data'
 * action to apply the membership at registration.
 * @see: https://rocketgeek.com/plugins/wp-members/docs/filter-hooks/wpmem_post_register_data/ 
 */
add_action( 'wpmem_post_register_data', 'my_set_basic_product' );
function my_set_basic_product( $fields ) {
    
    // Defaults.
    $product_meta = 'my_basic_product';
    $user_id = $fields['ID'];
    
    // Set product access.
    wpmem_set_user_product( $product_meta, $user_id );
}