How to embed custom HTML code with no plugins?

Sounds like you are using the Visual side of the editor.

enter image description here

You need to use the Text side.

enter image description here

Get it?

Update:

Preview won’t show embedded objects. Publish it privately if you absolutely must see it before anyone else, and need to see embedded bits like <object> tags.

Update #2:

Preview will show many embedded objects now as of WP 4.0. And many embeds can occur from merely a URL (tweets, youtube, for example).

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