You should start by creating your push campaign first in the Localytics Dashboard. You will need to note the Campaign ID that is set by the Localytics backend which is found in the URL during campaign setup. It is the number after "campaign/" in the dashboard URL. For example in the link below, the campaign ID would be 104272.
Next you will go and create your in-app message. You will need to set the campaign audience to send to "everyone", then finish your in-app creative and continue to scheduling. Under "scheduling", use the in-app trigger event "Localytics Push Opened" so that only users who have opened the app from the Push Notification see the creative.
Then add a filter to where "Campaign Name" is one of 104272 (this is the Campaign ID you got from the Push URL mentioned above and should be manually added.)
Next, activate the In-App campaign and let it run for a day. This will allow the users time to have their next app session in which the In-App creative will be downloaded to their device. Navigate back to the Localytics Dashboard, select your Push Campaign, confirm everything is correct and activate it.