Sprout Forms - Custom Form Notification Recipients With sproutForms.onBeforeSaveEntry
Based on the event you are referencing, I believe this is a question about Sprout Forms for Craft 2. In Sprout Forms for Craft 3, notifications are handled a differently.

In Sprout Forms for Craft 2, the event you'll want to use for this is the SproutForms_OnBeforePopulateEntryEvent.

You're not able to modify which value gets set directly, so you'll have to swap in the value you want using Craft Object syntax.

In your notificationRecipients field, add a value using Craft Object Syntax: {myRecipientsEmail}. Once you have called sproutForms.onBeforePopulateEntryEvent you can modify the $_POST data that gets processed when assigning the recipient value and it sure it has the value you want: $_POST['myRecipientsEmail'] = 'so-and-so@website.com'