Im making a subclass, and one of the abilities is to increase the range of attacks by 5 feet. How can I do this? Are there any preexisting abilities that I can base this off on?

I don't know if this is the sort of thing you are looking for but the Bugbear character race from Volo's guide to monsters has the following:

Long-Limbed. When you make a melee attack on your tum, your reach for it is 5 feet greater than normal.

This gives them 5' extra reach on melee attacks but only on their turn.

How can i integrate this in dndbeyond? Would it be a bonus or what?

The usual method to determine how DNDBeyond does something is to make a homebrew duplicate of something that has the ability and go through its features to find out how it's doing it.

In the case of Bugbear it adds:
Modifier Type: Bonus
Modifier Subtype: Melee Reach
Fixed Value: 5
All other fields blank.

Battlemaster fighter has a maneuver called “lunging Attack” that lets you spend a superiority die to extend your reach with a melee weapon attack by 5ft for 1 attack. On a hit attacks damage is also increased by that superiority does roll. Kind of weak if you ask me.