Pre-shared-key for VPN Site-To-Site

Hello guys !

I'm new to the VPN environnement, and trying to configure a site-to-site VPN tunnel between two Fortigate 60D.

At the step "Pre-shared-key", I don't know if it's a specific key to the device (in that case where can i find it ?) or it's a secret word that i can randomly create and share between the two fortigates.

I know that the PSK must be shared before any login/pasword authentification, but my question is:

Where to get the PSK from ? Or I can create it myself ?

Thanks !