On Tumblr, you can repost content from other users' blogs using the reblog feature. A reblogged post has certain markers, such as an indentation and a blog attribution, that differentiate it from original content. To add a custom link to another Tumblr account in your post, you can either reblog the post from the user's blog or insert the link manually.

Adding Name Links to Posts

When you reblog another user's post, Tumblr automatically adds a hyperlink to the blog of that poster. If that user originally reblogged the post from a third user, the link to the original source's blog appears in the footer of your post. You can also manually add a link to another user's blog by typing the username in the post form, highlighting it with your cursor and then clicking the chain-link icon in the post-formatting menu. Enter the URL of the blog to which you want to link and click "Insert Link" to insert it into your post.