The thing to do if one thinks a question should be put on hold/closed for being off-topic or otherwise unsuitable (while not being offensive or spam), but one does not yet have enough points to vote to close is to flag for its closure. (For details and commentary, see below.)
If the question already is on hold usually the only thing to do is to downvote it if one wants (except if one has enough points to vote to delete, in which case one coudl consider doing this). Closed questions with low score (including the answer score) are autodeleted quite quickly. To do this faster has some drawbacks. In particular, I do not think that to have a flag "off topic" with similar functionality as "spam" and "offensive" is needed or desirable. (The proposal seems to be about introducing this, not about improving it as the "other" flag is completely different in its effect.)
To use the "other" flag saying "off-topic" in such a case, seems a strange thing to do to me. This flag only informs the moderators and is completely different from the two other flags.
If at all I would flag with saying what I want, namely that the question be deleted. However, I think this is a misuse of a moderators flag. (It appears, however, that in a discussion about a year ago some assertion of a moderator of this site could be taken as encouragement for this type of action. So maybe they want this, though, it still seems strange to me.)
Below I give the details on the flagging dialog for open questions, as there is some confusion in the comments (note this is different for users that can vote).
Flagging for closure is done like this:
select "it should be closed for another reason"
select the more specific reason, which includes "off topic," but also "too broad", "primarily opinion based", "unclear what you are asking" and (again) "duplicate"
possibly some additional action, like choosing the sub-reason for "off-topic"
press "flag question"
As Stefan Kohl mentions it is also possible to flag as "very low quality" but this should not be done to a question that is written well but too simple to be on-topic; rather it is mainly to be used for posts that are "very low quality" for general reasons, like gibberish. (The consequences of this flag are somewhat different from the one mentioned above.)
The flag "other" plus explanation should not be used for signaling questions that are merely off-topic in a general way. This flag is the way to signal/communicate something to the site's moderator and should be used only if the matter requires the attention of a moderator specifically, that is when the general process of community moderation seems not capable of handling a particular matter.
One could imagine to streamline the above process a bit (it might be this was the purpose of the request), via exposing off-topic in the initial list (as is done for duplicate) for instance, but then the initial list is already somewhat long, too, so it is not clear what is preferable. And, the process of actually voting to close is not much simpler either. I thus do not see an urgent need or even clear benefit for saving a couple of clicks at this point while making the initial list slightly less handy.
It should however be possible to have a bookmarklet that saves some clicks; this exists for for votes to close (but I have not tried it), it should also be possible to have it for flags to close.