Hi vnat,
What I think is to create a Flag Quick Steps via "Create New" > "Flag Messages" if you are not using a IMAP account. Then when I was going to set a flag in 2 days I found that I can't custom here.

So it is suggested to directly select the messages and right-click to custom a 2 days flag. Remember to check Reminder.


As for the Quick Step for this, I will also submit this “Unable to customize” question internally. Before that, I'm afraid there is no other Quick Step to achieve this.
In addition, if you are using an IMAP account, there may be no such Quick step for this.
Hope this can be helpful.
Regards,
Aidan Wang
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