If your article has been accepted, feel free to cite this article as "forthcoming" (or "in press", although in the age of electronic publishing, "forthcoming" is the preferred style).
While we usually try to publish accepted articles in the current or upcoming issue, we cannot provide you with the exact bibliographic information (volume, issue, article number) at the time of acceptance, but the correct way to cite a forthcoming article is to use its DOI (Digital Object Identifier).
The DOI consists of a prefix identifying the publisher (10.2196) and the journal JMIR (for example, for JMIR it is 10.2196/jmir.) and your manuscript number:
Author name(s). Title. J Med Internet Res (forthcoming).
(note that the above URL linking to the so-called DOI resolver produces an error message at the time of acceptance before the paper is published, but immediately after the article is published (on the same day) it will point to the published article URL).
Using the DOI for in-press articles is a widely accepted standard, so even if you do not submit your follow-up paper to JMIR but somewhere else you should use this format.
If the JMIR paper has been accepted by the time your new citing papers is accepted, you should correct the "forthcoming" citation with the correct metadata (see How to cite JMIR articles? How to download them to my bibliographic management system?).
To find out the prefix for the journal it has been accepted for, please check the acceptance email (which contain the DOI) or check other articles in the same journal.