You can use options to customize this for every site. Options are stored separately for each site.
So, inside your header you can do something like
<?php
$login_link = get_option("worduoso_login_link");
$login_text = get_option("worduoso_login_text");
?>
and then print it like this
<?php if($login_link && $login_text): ?>
<div class="added-header"><a href="https://wordpress.stackexchange.com/questions/273810/<?php echo $login_link; ?>"><?php echo $login_text; ?></a></div><?php endif; ?>
So, how do you set this option?
If you want to add it just once you can just execute this code:
<?php
add_option( 'worduoso_login_link', 'http://www.example.com/login', '', 'yes' );
add_option( 'worduoso_login_text', 'Customer Login', '', 'yes' );
?>
(add code to functions.php and call the correct site, repeat for each site)
Better way is to add a section to theme customizer where those will be shown to admin under Appearance->Customize in WP admin. This is a bit more advanced code, here is the gist with 2 pieces – once to include in functions.php and one to include in header.php: https://gist.github.com/worduoso/9439aee7c5b91380527a06ac17498d41