oEmbed of video from alternative source not working

Site URL: https://www.uercportland.org/posters

'm trying to change video links on https://www.uercportland.org/posters from youtube (hosted by a contractor) to my university's Kaltura media server. SS instructions say to use oEmbed code, which Kaltura generates and I paste into the video link via the </> option. However, I don't see any "Set" button as described in the instructions (just the Back option - see attached screenshot). I click Back (or just away from the code) and save the page, but the link is not updated in the live site (still youtube). When I go back to edit the embed code, it has been altered to html preformatted text (see Original & Changed code blocks below). This code is for the first poster (Birdsong). I've tried both Chrome & Edge browsers.

Original oEmbed code:

<iframe id="kaltura_player" src="https://cdnapisec.kaltura.com/p/1829851/sp/182985100/embedIframeJs/uiconf_id/44585072/partner_id/1829851?iframeembed=true&playerId=kaltura_player&entry_id=1_nybf4bad&flashvars[streamerType]=auto&amp;flashvars[localizationCode]=en&amp;flashvars[hotspots.plugin]=1&amp;flashvars[forceMobileHTML5]=true&amp;flashvars[Kaltura.addCrossoriginToIframe]=true&amp;&wid=1_voq8fi2x" width="400" height="285" allowfullscreen webkitallowfullscreen mozAllowFullScreen allow="autoplay *; fullscreen *; encrypted-media *" sandbox="allow-forms allow-same-origin allow-scripts allow-top-navigation allow-pointer-lock allow-popups allow-modals allow-orientation-lock allow-popups-to-escape-sandbox allow-presentation allow-top-navigation-by-user-activation" frameborder="0" title="Kaltura Player"></iframe>

Changed oEmbed code:

<pre class="source-code"><div>&lt;iframe id="kaltura_player" src="https://cdnapisec.kaltura.com/p/1829851/sp/182985100/embedIframeJs/uiconf_id/44585072/partner_id/1829851?iframeembed=true&amp;playerId=kaltura_player&amp;entry_id=1_nybf4bad&amp;flashvars[streamerType]=auto&amp;amp;flashvars[localizationCode]=en&amp;amp;flashvars[hotspots.plugin]=1&amp;amp;flashvars[forceMobileHTML5]=true&amp;amp;flashvars[Kaltura.addCrossoriginToIframe]=true&amp;amp;&amp;wid=1_voq8fi2x" width="400" height="285" allowfullscreen webkitallowfullscreen mozAllowFullScreen allow="autoplay *; fullscreen *; encrypted-media *" sandbox="allow-forms allow-same-origin allow-scripts allow-top-navigation allow-pointer-lock allow-popups allow-modals allow-orientation-lock allow-popups-to-escape-sandbox allow-presentation allow-top-navigation-by-user-activation" frameborder="0" title="Kaltura Player"&gt;&lt;/iframe&gt;</div></pre>

OK I found the problem. I was trying to edit an existing "video" block (containing a youtube link). Instead I needed to create a new "embed" block, which accepted the embed code, had the Set button, and works.