I have created a Homebrew weapon called a Strongbow; essentially a heavy-pull longbow that allows the wielder to add their strength bonus (up to +5) when determining damage.

Where I run into trouble is getting the Homebrew weapon to properly add the damage, I was hoping 'Bonus - Magic Item Attack With Strength' would do this, but I was mistaken.

Searching turned up a few promising leads, but none that panned out.

Can anyone point me to were I might be able to codify this into a homebrew creation?

You mean something like this where it uses Str instead of Dex: (https://www.dndbeyond.com/magic-items/1243170-heavy-longbow)? Or do you want to add Str to the damage roll in addition? If the latter, then try this:

Modifier: Damage->Subtype Piercing->Ability Score: Str

You will still have to customize the attack on the character sheet to add the additional damage, but that modifier will at least act as a reminder to do it.