Adding a 1d8 radiant damage to a magic weapon is fairly straight forward:
- Add a modifier
- Set Type to Damage
- Set Subtype to Radiant
- Dice count to 1
- and Dice Type to d8
This should make the d8 radiant damage be added as a note on the attack option.
However, this doesn’t make it automatically rolled when you roll for damage (this might have something to do with dice and mixed damage types). There is a workaround for this though, but it only works when the damage type doesn’t matter and you’re not using the versatile option. Set up a custom attack action.
Hit manage custom, and create a new action option. You’ll want to set Range to Melee, Stat to STR, and Activation type to Action and mark Proficient and Display as Attack. Then set Dice Count to 2 and Dice Type to d8. You can use Fixed Value (to the right of the dice values) to add a static bonus to the damage roll, such as from Dueling. And give it a name so you remember what the attack option is. This will at least give you a conveniently accessible 2d8 + STR damage roll.