Checking the tumblr API I don´t think there is not a request parameter to add a source_url for the content.

For example creating a photo. The source parameter refers to the source for the picture, but the source_url or "content source" is actually the original source for the content. Which is explained here:

https://support.tumblr.com/post/125279368827/heres-a-nice-thing-no-matter-how-many-times-an

It will be nice to be able to add that directly from the API when creating a new post.