Facebook Messenger Alert Sound

How do I change the alert sound for Facebook’s Messenger on my iPad? Going through settings>notifications, the app only gives the option to have sound on or off, not to change it. In the Settings>Sounds section there is no specific mention of Mesenger, only emails, reminders etc. It would be really handy for my husband and I to have a different messenger sound for our two iPads as it’s confusing both sounding the same!