define( 'WP_HOME', 'https://mydomain.com/blog' );
define( 'WP_SITEURL', 'https://mydomain.com/blog' );

Update .htaccess File

Your current .htaccess file looks almost correct, but it may need to be adjusted for Azure's Linux environment.

Try this configuration:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /blog/
RewriteRule ^index\.php$ - [L]

Redirect everything else to the correct subdirectory.
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /blog/index.php [L]
</IfModule>
# END WordPress

Check Azure App Service Path and Folder Structure

Ensure the folder structure is correctly set up on Azure App Service:

• WordPress files should be inside the /site/wwwroot/mysite/blog folder.
• Confirm that your Azure app is running PHP properly, and all necessary PHP extensions for WordPress are enabled (e.g., mod_rewrite for URL rewriting). 

define( 'WP_ALLOW_MULTISITE', true );
define( 'MULTISITE', true );
define( 'SUBDOMAIN_INSTALL', false );  // Set to false for subdirectories
define( 'DOMAIN_CURRENT_SITE', 'mydomain.com' );
define( 'PATH_CURRENT_SITE', '/blog/' );  // Ensure this is set correctly
define( 'SITE_ID_CURRENT_SITE', 1 );
define( 'BLOG_ID_CURRENT_SITE', 1 );

File Permissions

Make sure that the file permissions are correct for the /blog directory and its contents:

• Use FTP or Azure CLI to ensure files have the correct permissions (e.g., 755 for directories and 644 for files).

Check that the wp-config.php file and other critical files have the appropriate permissions.