Getting the inactive users list in segment is completely based on your App requirement. It depends on the facts that how a user can be said as inactive. For example: In some apps, developers calculate inactive users based on App sessions and some of the developers calculate based on activity performed inside the App. We recommend you to calculate inactive users based on the first case as follows:
Create two segments one for App Install and second for App session's start event
At the time of creating segment, you need to select the date range, event, and properties for which you want to target
Now jump to Push campaign (Campaign -> Push), create your new push campaign, select existing segment and there are two options:
User who have done - Select segment which is created for Install event
User who have not done - Select segment which is created for App Session
Now, enter the rest of fields and you are all set for the campaign. Once your app users fall into this segment, they will receive the push automatically which you have configured through the dashboard.
P.S.: You can also send message to these segmented user through on-demand targeting.
Please have a look at dashboard and let me know if you need any help while integrating it.
I will be Happy to assist you!!!