So I made a home brew weapon, nothing too fancy, it’s a war hammer (1d8) I was wondering if there’s a way to have it do an extra 1d4 bludgeoning damage and reflect that additional roll when click the damage roll button on my character sheet? Ie, it rolls a 1d8 and 1d4 simultaneously.
If so what would the steps needed to do it correctly? 
thanks in advance 

Greetings SlimyLilDevil,

As far as I know, there is not a way to change the Damage Dice for pre-determined Weapons, and those used for Magic items.

If you customize a description to say "+1d4 Bludgeoning" it will show up in the item description, but will not be a rollable button.

I know you can create custom items (non-magic), but I do not believe you can have two different types of dice for damage there either.

Cheers!

I checked how flametongues work, and they don't automatically add the 2D6, so I highly doubt there is a way to do this.

Yeah, there isn't. You can add a flat number as a damage bonus, but not a die roll.

Quote from SlimyLilDevil >>

So I made a home brew weapon, nothing too fancy, it’s a war hammer (1d8) I was wondering if there’s a way to have it do an extra 1d4 bludgeoning damage and reflect that additional roll when click the damage roll button on my character sheet? Ie, it rolls a 1d8 and 1d4 simultaneously.
If so what would the steps needed to do it correctly? 
thanks in advance 

 
There is no way to auto-roll the extra damage on D&D Beyond. The person using the weapon can create a custom action for the 1d4 to make it easier to roll that after rolling the normal damage. Here is a crappy video I made to show how to do this: