Maniphest tasks offer the option to "add a subtask" (which will block the current task) but not the option to "add a follow-up task" (which will be blocked on the current task).
The only way to specify such a task currently is to independently create a task, then search and add the original one as a subtask. There is no one-click action like "add a subtask". There is no parameter in URLs to create follow-up tasks (the way parent= lets you directly create a subtask). The Conduit API doesn't let you specify such a relationship either.
This is a UX issue, in particular for projects that use lots of follow-up tasks. That's the case for any project maintaining stable branches for backports. A task might be a bugfix in the master branch, and you want to file a follow-up task to backport that fix to a stable branch once the master fix is in.