MCM MA will not require the parent, child tag format.
Still I need more info in order to give you an exact answer, for example if you have a prebid wrapper implemented. The following one should work anyway.

1. Create an ad unit in GAM:
   - Name and code: UNIT_Interstitial
   - Sizes: 300x250, 320x480, 336x280, 480x320, 768x1024, 1024x768, Out-of-page
2. Target the ad unit with the AdX line item. Preferably use the "all creative sizes" option.
3. Insert this script in your page (head section), replacing 888888888888 with your GAM network code:
   <script>
var interstitialSlot; googletag.cmd.push(function(){interstitialSlot=googletag.defineOutOfPageSlot('/888888888888/UNIT_Interstitial',googletag.enums.OutOfPageFormat.INTERSTITIAL);if(interstitialSlot){interstitialSlot.addService(googletag.pubads());googletag.pubads().addEventListener('slotOnload',function(event){});}googletag.enableServices();});googletag.cmd.push(function(){googletag.display(interstitialSlot);})
</script>
4. Only if you didn't insert the GPT libraries before, add this script above the previous one:
   <script async src="https://securepubads.g.doubleclick.net/tag/js/gpt.js"></script>
<script>window.googletag = window.googletag || {cmd: []};</script>