I don't think we have this capability right now.

https://secure.phabricator.com/book/phabricator/article/custom_fields/ custom fields allow you to set the required property in configuration. I'm not sure if you can use this to override a default built-in field but you could possibly replace the built in field with a custom field with same/similar name and type: users.

That would look something like this:

{
 "mycompany:assigned-to": {
    "name": "Assigned-to",
    "type": "users",
    "required": true,
    "limit": 1
  }
}

This wouldn't really work like the built in field, however, since the built-in field presumably has some custom functionality.

If you are comfortable editing the PHP source to phabricator then it should be pretty trivial to add the required property to the built in field implementation but I don't know off the top of my head where you would find it.

One thing that could be done right now is:

Create a new Herald rule (Global, Maniphest Tasks) that, if the Assignee does not exist, it runs an Action "Add comment", with this content:

You can also add a new Tag like #task-without-assignee so to highlight the issue. And you can automatically remove that tag with another Herald rule.

If you are really really really interested in hard-blocking Task creation from custom rules using Herald, it should be probably developed a new Herald Action to support the "block Task creation" if something happens. But I honestly cannot implement this feature, so at the moment I hope the "auto-comment" is enough for you.

In T15101#3866, @valerio.bozzolan wrote:

One thing that could be done right now is:

Create a new Herald rule (Global, Maniphest Tasks) that, if the Assignee does not exist, it runs an Action "Add comment", with this content:

IMPORTANT: Please set the Assignee! Or a kitten will die RIGHT NOW!

You can also add a new Tag like #task-without-assignee so to highlight the issue. And you can automatically remove that tag with another Herald rule.

@SmoothPlay: Is that proposed workaround totally silly, or could it make sense for you?