Topic on Manual talk:$wgFooterIcons

This is not documented and acutally I cannot find this parameter in the codebase either but it works:

$wgFooterIcons['poweredby']['exampleorg'] = [
    'src' => "$wgScriptPath/images/badge.png",
    'url' => 'https://example.org/',
    'alt' => 'Example Organization',
    'style' => 'margin-left: 5px;'
];

Perhaps this is useful to know for others, too.