Add native asset placeholders section

adops/setting-up-prebid-native-in-dfp.md

@@ -42,7 +42,7 @@ Select **New format** under **Custom format**. (If you've already created an ad
 
 ## 3. Style your native ad
 
-You can add HTML and CSS to define your native ad template. To allow for native impression trackers and click trackers within a Prebid native creative template, you'll need to include a CDN-hosted script in the HTML, as shown here (see Example HTML below for the full script):
+You can add HTML and CSS to define your native ad template. To allow for native impression trackers, click trackers, and [automatic placeholder value replacement]({{site.github.url}}/dev-docs/examples/show-native-ads.html#sending-asset-placeholders) within a Prebid native creative template, you'll need to include a CDN-hosted script in the HTML, as shown here (see Example HTML below for the full script):
 
 ![native ad styling]({{site.github.url}}/assets/images/ad-ops/dfp-native/prebid_native_styling.png){: .pb-md-img :}
 

dev-docs/show-native-ads.md

@@ -204,6 +204,73 @@ const adUnits = [{
 </div>
 {% endhighlight %}
 
+## Sending Asset Placeholders
+
+Prebid.js sends received asset values to a native template defined in your ad server using key-value targeting. The key-value targeting pairs are passed to the ad server as query string parameters. In some cases, sending native asset values as query string parameters may cause errors. For example, a long `clickUrl` value can exceed an ad request URL limit, or special characters within a `body` can get mangled by URL encoding. In these cases, you can opt to send URL-safe placeholder values to the ad server, and then in the native template, replace the placeholder values with the actual native values through a non-URL request to and response from Prebid.js. Here's how it works:
+
+### 1. Configure which assets to send as placeholders
+
+Within `mediaTypes.native`, add `sendId: true` to any asset object you wish to send as a placeholder. For example, to send `body` and `clickUrl` as placeholders:
+
+```javascript
+mediaTypes: {
+  native: {
+    body: {
+      sendId: true
+    },
+    clickUrl: {
+      sendId: true
+    },
+  },
+},
+```
+
+### 2. Prebid.js sends placeholder values
+
+For the assets configured with `sendId: true`, Prebid.js sends a placeholder value describing the asset type and the `adId` for the associated ad. In this example, Prebid.js would send the body and clickUrl assets as values like `hb_native_body:38a33cef3e926f` and `hb_native_linkurl:38a33cef3e926f`. The value after the `:` in each of these examples is the `adId` generated by Prebid for that native ad object.
+
+### 3. Native template replaces the placeholders
+
+Finally in the native template on the adserver, search for any expected placeholder values and replace them with their actual values.
+
+Placeholder values will start with the `hb_native_...` key for that asset type, contain a `:` separator, and end with the `adId` for that ad object (available via the Prebid targeting key `hb_adid`). When Prebid.js receives a postmessage request in the form
+
+```json
+{
+  "message": "Prebid Native",
+  "action": "assetRequest",
+  "adId": "38a33cef3e926f",
+  "assets": [
+    "hb_native_body",
+    "hb_native_linkurl"
+  ]
+}
+```
+
+it will respond to that request with the actual asset values for that `adId` in the form
+
+```json
+{
+  "message": "assetResponse",
+  "adId": "30655b7d68dc51",
+  "assets": [
+    {
+      "key": "body",
+      "value": "This is a Prebid Native Creative"
+    },
+    {
+      "key": "clickUrl",
+      "value": "http://prebid.org/dev-docs/show-native-ads.html"
+    }
+  ]
+}
+```
+
+A script within the native template can listen for this response and replace the placeholder values with their actual values.
+
+The `native-trk.js` script available as part of `prebid-universal-creative`, if used in a template that sets any link with a `pbAdId` to `hb_adid`, will replace any placeholder values with their actual values automatically (note: this works for placeholders located within HTML native templates but not CSS native templates). See [Style your native ad]({{site.github.url}}/adops/setting-up-prebid-native-in-dfp.html#3-style-your-native-ad)
+for more on how to set this up in a native template.
+
 ## Working Examples
 
 + [Prebid Native with two slots]({{site.github.url}}/examples/native/native-demo.html)