My character is a Hexblade Warlock and DM and I were planning on giving him a weapon that has to deal with him personally. I love the process of making it but it seems to be difficult. I want to be able to add my charisma modifier for damage and to hit, but I can't seem to find a way to make it increase as my charisma modifier does. This goes along with wanting to remove the strength modifier. Also, it being a Longsword, it does 1d8 slashing. I wanted to make this a bit less to balance it out, because I don't want to be overpowered. Is this available in D&D Beyond yet? If so, how could I change these values? Any and all help is appreciated!

I'm just going to unpackage this request really quick; you need to do the following:
     *Use Charisma instead of Strength for Attack and Damage
     *Reduce the standard die of the Longsword

First thing's first, Hexblades get to use their Charisma instead of their Strength or Dexterity automatically as a class feature - no editing required.  See the entry "Hex Warrior", they get it at level 1 - all you have to do is click on the weapon Action on the character sheet and check the box "Hex Weapon" to make it active...unless you're wanting to grant him a new, unbound weapon that uses Charisma instead of Strength, with which there are ways around that, too.
The easiest way to do so would be to just mark it as your "Hex Weapon" on the DnDBeyond character sheet - that will change the modifiers for you right there and the editor allows you to mark multiple items with the box.
If this is something you want other players to potentially be able to access and use, this will require a bit more work but I can see two ways of doing it:
     1)  You're able to make Custom Actions where you can set the name of the weapon, the damage type, damage die, and the ability score it derives it's attack and damage from.  This will place it in the "Other Actions" tab of your Actions list.
     2)  Create a new Spell and attach it to the Weapon, then you can use the "Spell" in place of the Weapon attack.  Just set the Spell's Damage to be what it would be for the Longsword (you may want to make two since Longswords are Versitile, but that's completely up to you and your DM).

Which brings me to the next point - since Hexblades get to use their Charisma instead of Strength already as a class feature, I don't think that lowering the damage die of the Longsword is necessary.  If you DID decide to reduce it anyway, you may want to just use a different kind of weapon and just call it a Longsword by editing the customization on the character sheet.

Hope this helps!