You have two options; either bake into the theme (functions.php) or write a simple plugin that auto-creates the post based on hooking into after new site (blog) creation:
https://codex.wordpress.org/Plugin_API/Action_Reference/wpmu_new_blog
Use the insert post function to do the creation:
https://developer.wordpress.org/reference/functions/wp_insert_post/
Then, set as the home page based on ID returned:
update_option( 'page_on_front', $new_page->ID );