Please advise - Trying to embed user login block - getting invalid block error when creating source

I'm trying to embed the user login in an external, plain html page on a given site. (Lot's of peculiar reasons not worth going into here).

I've checked the

Widget Information
Provide this block as a source to be embedded in widgets.

box on the login block config page

I can't seem to configure an iframe widget to export that block no matter what I do.

1) the "add a source" throws me to the create widget tab, which doesn't present the login block in the blocks dropdown.

2) I can't see any way to use the pre-built source based on the login block.

3) I can't find what code to insert in the external page anywhere.

I'm somewhat new to all this but not completely ignorant. However I'm baffled by what I'm seeing here.

Please advise.