How do I add a value to a tag field type using the entries service saveEntry method?
The Tags field classifiedTags needs an array of tag IDs, not slugs/titles. Also, if this is an existing entry and you want to retain tags already added to the field, you'll need to pull the existing tag IDs using $entry-&gt;classifiedTags-&gt;ids() and merge with those.

$tag = craft()-&gt;elements-&gt;getCriteria(ElementType::Tag, [
    'groupId' =&gt; 1,
    'slug' =&gt; 'tagtest',
])-&gt;first();
if ($tag) {
  $existingTags = $entry-&gt;classifiedTags-&gt;ids();
  $entry-&gt;getContent()-&gt;setAttribute('classifiedTags', array_merge($existingTags, [$tag-&gt;id]));
}


Note that if the tag doesn't already exist, you'll need to create it using the TagsService:

$tag-&gt;groupId = 1;
$tag-&gt;getContent()-&gt;title = 'tagtest';
$success = craft()-&gt;tags-&gt;saveTag($tag);