I know this is a silly question, but I have not found the easy answer in the Docs or Forum (probably because it's so basic).
Where do I launch the iFrame dialogue panel to add an iFrame to my second server webpage?
The docs start from when the dialogue is launched but not how to launch it.

I looked in the optional plugin downloads as some of the posts hint at that...but none found.

First you have to add the button to the toolbar in your profile.

Please see: https://www.joomlacontenteditor.net/support/faq/plugins/iframe-button-is-not-in-the-editor-layout-after-install