How and where to create a prefix key for memcache?
All those settings are added to settings.php

usually sites/default/settings.php

$conf['cache_backends'][] = 'sites/all/modules/contrib/memcache/memcache.inc';
$conf['cache_default_class'] = 'MemCacheDrupal';
$conf['memcache_key_prefix'] = 'k1';
// Use database for cache_form
$conf['cache_class_cache_form'] = 'DrupalDatabaseCache';


You only need a memcache key prefix if you're using memcache with more than one site on that server. Adding a short key, e.g $conf['memcache_key_prefix'] = 'k1'; is good practice.