Not all the SE sites are in the option when flagging a question as off topic. It would be nicer to suggest the exact SE site when flagging off-topic questions. Or am I missing something? Is such an option available in addition to notifying it in the comments section?
There are two ways for migration:
(Flagging →) Closing → Off-Topic → Migration – This is only available for frequent migration targets. Academia has only its own Meta as target as this is activated by default.
Adding new migration targets here is possible in theory, but as far as I can tell, there is no site to which we are migrating very often. Most questions we get that are on-topic on another site are still closeworthy there (due to being unclear, opinion-based or similar) and thus they should not be migrated. Also such a target should only be established if close voters can be expected to have a clear idea as to what is on-topic on the target site, which does not hold here in my opinion.
Also note that if you flag for closure (and do not have the close privilege), the only effect closure has is that the close voters see this and may thus need less time to find out what shall be wrong with the question.
Migration by moderator – If you are sure that a post needs at most minor tweaks to be a good fit for another site, you can use a custom flag to alert a moderator to migrate it.
In this case, you can also flag to close. While custom close reasons exist for users with the close privilege, they are not available to flaggers (see here). So, all you can do is flag as blatantly off-topic, leave an explaining comment.
Two side notes:
- If you think a question is generally a good fit for another site, but has issues, advise the asker to improve these issues and read the other site’s guidelines.
- If you are not sure whether a question is a good fit for another site, state this in your comments and advise the asker to check the other site’s guidelines (or do so yourself). A considerable amount of users get frustrated because they are recommended other sites when their question does not actually fit there (we have this problem ourselves, see here).
To suggest migration to another site, you have to add a custom comment. I was confused by this myself at first, but apparently this is a deliberate choice by design due to the very large number of SE sites that now exist (see this meta question for some history).
I would recommend adding a comment manually, and then using the "Blatantly off-topic" flagging reason.