How do I embed a YouTube video?

Preface

I have tried to embed a YouTube video, many times.  I am not asking, "Where do I do this?"  Nor am I asking, "How do I get the embed-code?"

I have the iframe to insert (which you can't see properly, because HTML-encoding the URL characters¹ gets reverted on [POST]):

<iframe width="560" height="315" src="https\:\/\/www\.youtube\.com\/embed\/yEcvwuAH-D8" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>

I have already tried the following:

1. Insert➔External Content➔YouTube Video
   1. After pasting the iframe...
      1. clicking "OK" doesn't do anything.
      2. clicking "Cancel", however, works fine.
2. Tools➔<> Source Code

Questions

1. Does anyone reading this have experience with this issue?
2. Did you embed the iframe successfully?
3. How?

¹ It displays properly in Tools➔Preview.  But, when I post this — the URL gets converted into a broken link.  I only got the horizontal scroll, because I edited the source to use "pre" tags — instead of "p" tags.  When I try to use Insert➔</> Code, I get them same behavior I describe for Insert➔External Content➔YouTube Video.