You don't, if you want to embed something in a post, you have 2 options:

• oembed: Great for stuff like youtube, and other things, think of embeds, think of things that can be represented as URLs, e.g. copy paste a youtube URL on to a blank line and it turns into a youtube player
• shortcodes: This is how you run PHP code and insert HTML inside a post

Anything that will let you copy paste a form as is into a post edit screen is going to be a security problem, dramatically increasing the chances of your site becoming hackable

You don't, if you want to embed something in a post, you have 2 options:

• oembed: Great for stuff like youtube, and other things, think of embeds, think of things that can be represented as URLs, e.g. copy paste a youtube URL on to a blank line and it turns into a youtube player
• shortcodes: This is how you run PHP code and insert HTML inside a post
• filters: You can append the code via the_content filter to your posts when they're rendered. This allows you to add things without modifying the post content

Anything that will let you copy paste a form as is into a post edit screen is going to be a security problem, dramatically increasing the chances of your site becoming hackable