You want to embed a canvas app in a model driven app, but you want to start with one that you have already built, not create one from the embedded canvas app control.
Go to powerapps.com and click the (…) by the app you wish to embed. Click Details, then find and copy the App ID.
On the D365/model-driven app form, add the embedded canvas app control where desired and click the configure property button. Paste the App ID copied in step 1.
Click “Customize” to launch your canvas app in PowerApps Studio. This will add the ModelDrivenFormIntegration component to your existing canvas app. Don’t forget to enable the embedded canvas app experimental control.