400 Bad Request when crosspost video & audio post formats

Some things I’d check first are:

• When writing a new post, have you selected the correct Post Format before pressing “Save draft” or “Publish”? Note that the default Post Format is “Standard” (i.e., text), and once a post or draft is saved, Tumblr will not allow you to change the format (although WordPress will).
• Is a supported video or audio link present somewhere in the WordPress post body? That is, if you are crossposting a YouTube video, you need to ensure that the URL to the video exists somewhere in the WordPress post body. This can be a textual, clickable link, or it can be an <embed> code, but it must be present in the WordPress post proper for Tumblr Crosspostr to detect, extract, and cross-post it. Same with audio media.

Additionally, there’s probably more information in your WordPress’s wp-content/debug.log file. Consider activating “Detailed debugging information” in the Tumblr Crosspostr settings (and the WP_DEBUG directive in your WordPress config file) to get more information about what’s going wrong.

Ah, got it. I need to use <iframe> within the body/content of my wp-post to make video/audio format work at Tumblr. Somehow, the plugin cannot process Youtube or any other WP-compatible video/audio-file hosting services if the link is still in its raw shape or any other ones, i.e. [embed], etc. Hence I need to put the link inside <iframe> and put display:none as the inline CSS.
Anyway, thank you!

Correct. The most reliable way to ensure correct cross-posting is to use raw <iframe> HTML. Tumblr Crosspostr may be able to detect the use of some shortcodes from other plugins but this is very unreliable as Tumblr Crosspostr can not know when (during a page’s processing pipeline) other plugins may change the content of the post. To avoid this issue, use HTML.