Change the Source DB for migrating via Drush
in settings.php add the connection to the source database (that contains users you want to migrate)
$databases['myuserssourcedb']['default'] = [
  'database' =&gt; '',
  'username' =&gt; '',
  'password' =&gt; '',
  'host' =&gt; '',
  'port' =&gt; '',
  'driver' =&gt; '',
  'prefix' =&gt; '',
  'collation' =&gt; '',
];

in your migration yml file reference the source database as 'key' in the 'source'
id: migration_of_users
label: 'Migrate users from old database'
migration_group: migration
source:
  plugin: mypluginforusers
  key: myuserssourcedb  &lt;-----This is the important line
process: