Create category on WPMS blog from another blog in the same network.

gistfile1.php

<?php 
// current blog
    $switch_from = get_current_blog_id();
// blog id we are switching to so we can create a category there
    $switch_to = 2;
    
    echo 'About to switch to '.$switch_to.' from '. $switch_from.'.';
    
// https://codex.wordpress.org/WPMU_Functions/switch_to_blog
    switch_to_blog( $switch_to );
    
    echo 'You switched from blog ' . $switch_from . ' to '.$switch_to;

// https://codex.wordpress.org/Function_Reference/wp_insert_term
	wp_insert_term(
	  'monkeys', // the term 
	  'category', // the taxonomy
	  array(
	    'description'=> 'Very fond of bananas.',
	    'slug' => 'monkeys'
	  )
	);

// https://codex.wordpress.org/WPMU_Functions/restore_current_blog
    restore_current_blog();
    echo 'You switched back to '.get_current_blog_id().'.';
?>