bestow curse damage and magic mislle and eldritch blast.

how does it work together? would it make it so eldritch blast does 2d8+1d10+cha with agonizing blast?

Quote from SunIsGettingRealLow96589 >>

https://www.dndbeyond.com/spells/2618923-bestow-curse

"the target suffers =one= of the following effects of your choice:"

"If you deal damage to the target with an attack roll or a spell, the target takes an extra 1d8 Necrotic damage"

Eldrichtch blast: 1d10

Agonizing Blast +Chamod

Bestow curse: +1d8

So 1d10+1d8+chamod

Hex or hunters mark is only a +1d6 but is also only a level1 slot, so probably better bang for the buck...

 yes, but it is a spell and an attack roll. how does that apply?

Quote from atomicgamer2012_13 >>

Quote from SunIsGettingRealLow96589 >>

https://www.dndbeyond.com/spells/2618923-bestow-curse

"the target suffers =one= of the following effects of your choice:"

"If you deal damage to the target with an attack roll or a spell, the target takes an extra 1d8 Necrotic damage"

Eldrichtch blast: 1d10

Agonizing Blast +Chamod

Bestow curse: +1d8

So 1d10+1d8+chamod

Hex or hunters mark is only a +1d6 but is also only a level1 slot, so probably better bang for the buck...

 yes, but it is a spell and an attack roll. how does that apply?

 Is it a spell or an attack roll? Yes.

Therefore 1d8 damage.

The spell doesn't say "1d8 damage each for damaging it with a spell or an attack roll". It sets a condition with multiple enabling possibilities, and if you fill any one of them, the condition is true. This is extremely normal English usage.

Since you also mentioned Magic Missile in the thread title, it's worth pointing out that the extra damage applies to that too. While it doesn't involve an attack roll, it is still you "dealing damage with a spell". 

The interesting question would be: if you hit the cursed target with more than one dart from Magic Missile, does each dart deal 1d8 extra damage? I'm inclined to think it does, as each dart is a separate instance of "dealing damage" and there's nothing in Bestow Curse that limits the effect to "once per turn" or anything like that.