Add this filter to the query.
Due to the nature of fapi, the value and the operator have an unintended level of indirection. You will find them in $this->operator and $this->value respectively.
Overrides PluginBase::query
public function query() {
$this
->ensureMyTable();
$this->query
->addWhere($this->options['group'], "{$this->tableAlias}.{$this->realField}", $this->value, $this->operator);
}