Users are not allowed to add HTML to their posts. This includes iframes. There is no functionality to allow for this and there is no plans to allow for this. iframes are considered a vulnerability. Typically you'd just use the links feature if you need to link to something external.

Only way HTML is allowed is if the post is a system post (click the anonymous person icon to the left of Post button), but iframes will still get filtered out unless you whitelist them for all of CB within CB > Configuration > User Profile using "Exclude Tags from Filtering" parameter. Don't do this if you're using editor fields as then users can also add iframes in those fields. There is no functionality in CB Activity at this time to add whatever unfiltered HTML you like. It maybe allowed in a future release for system posts only.

Safest way to add support for your iframe is to extend the parsing API with a new shortcode. This can be done using the below in CB Auto Actions.

Global
Triggers: activity_onParse
Type: Code
User: Automatic
Access: Everybody
Action
Method: PHP
Code:Parameters
Reference Variables: Variable 1

So with the above SHORTCODE_HERE for example could be [vod_partner] which will get replaced by whatever you add in place of IFRAME_HERE when you add [vod_partner] to your post. This will allow you to safely insert custom HTML. Note you'll need latest build released today for this to work as there was a bug in that trigger.