RAF implementation

Hello. I am new to RAF and ad implementation. I would like to know from where do we get this VAST url which we can provide inside the setAdUrl()?
Can anyone help me in clearing my doubt and is there any procedures that we have to follow to successfully implement RAF in my channel to show pre-roll, mid-roll and post roll ads?

As per the Roku Doc here, if your application is using Roku’s default ad server to fill ads, no other configuration is required. Just leave it as:
adIface = Roku_Ads()
adIface.setAdUrl()

Otherwise, you would need to add the URL of your ad-server to configure the ad URL before making the ad request call.

More details here.