In the app you want to "keep alive" simply subscribe to one of the events published by WakeUp according to your requirement:

• Ping_15: Triggered every 15 minutes
• Ping_Hourly: Triggered every 60 minutes
• Ping_Daily: Triggered every 24 hours (by default at 8:00AM)

Select your most suited event, define a handler action and you're good to go.

The scheduling of the triggers can be adjusted in the WakeUp app timer settings.