How to get Facebook post ID from ad using API?

I'm very happy with the Facebook Ads reporting tool I built myself.

It lists all campaigns in an ad account, and all ad sets in each campaign, and all ads in each ad set.

It shows the metrics I care about.

But I haven't been able to figure out how to retrieve the post ID from the ad.

It seems that this field is not mentioned in Insights edge ( not available in Insights InsightsFields), so maybe I have to "join" on another API call.

My main code for getting insights is:

public function getInsights($levelType, $id, $aggLevel, $start, $end) {
    if ($levelType) {
        if ($id == null) {
            abort(500, 'You must provide the ID for the object you want to retrieve.');
        }
    } else {
        $levelType = \AdAccount::class;
        $id = ACT_PREPEND . $this->fbConfig['account_id'];
        $aggLevel = AdsInsightsLevelValues::CAMPAIGN;
    }
    $variableClassWithNamespace = '\FacebookAds\Object\\' . $levelType;
    $level = new $variableClassWithNamespace($id);
    $fields = [
        InsightsFields::CAMPAIGN_ID,
        InsightsFields::CAMPAIGN_NAME,
        InsightsFields::ADSET_ID,
        InsightsFields::ADSET_NAME,
        InsightsFields::AD_ID,
        InsightsFields::AD_NAME,
        InsightsFields::SPEND,
        InsightsFields::UNIQUE_IMPRESSIONS,
        InsightsFields::INLINE_LINK_CLICKS,
        InsightsFields::INLINE_LINK_CLICK_CTR,
        InsightsFields::COST_PER_INLINE_LINK_CLICK,
        InsightsFields::ACTIONS,
        InsightsFields::COST_PER_ACTION_TYPE,
        InsightsFields::CPM,
    ];
    $params = [
        AdReportRunFields::LEVEL => $aggLevel,
    ];
    if ($start) {
        $params[AdReportRunFields::TIME_RANGE]['since'] = $start;
        if (!$end) {
            $params[AdReportRunFields::TIME_RANGE]['until'] = (new \DateTime("+2 year"))->format('Y-m-d');
        }
    }
    if ($end) {
        $params[AdReportRunFields::TIME_RANGE]['until'] = $end;
        if (!$start) {
            $params[AdReportRunFields::TIME_RANGE]['since'] = (new \DateTime("-1 year"))->format('Y-m-d');
        }
    }
    if (!$start && !$end) {
        $params[AdReportRunFields::DATE_PRESET] = InsightsPresets::LIFETIME;
    }
    $insights = $level->getInsights($fields, $params);
    return $insights->getResponse()->getBody();
}

So I'm at least willing to answer this simple question: Given an ad ID, how do I retrieve the ID of the "post" to show in that ad, assuming I have all the required API permissions (which I certainly do)?

Ideally, what I would most like to answer is: How do I retrieve that Post ID in one API query when retrieving all the insights I need (as shown above)?

Thanks!