[SalesForce] Can we Enable Send Log Per Business Unit in Marketing Cloud

There's a hidden column named _CustomObjectKey in every Data Extension, if that helps. There's more info on it here on the SFMC wiki page.

There's nothing stopping you from inserting any kind of value you want into the SendLog at send time with AMPScript, as long as it's meaningful to you. You don't necessarily need to make it a primary key to retrieve and update values in a specific row.

I probably would not recommend updating column values in the SendLog. They're generally huge and unwieldy.

The Send Logging feature needs to be enabled in your account for this template to be available. So you need to reach out to your account executive or log a new case in salesforce help & training portal where you ask for "Send Logging feature activation".

Your account must use the send logging and data extension features. Contact your relationship manager for more information about enabling these features.

(Source: Marketing Cloud documentation on Send Logging)

Further reading:

• Unleash the power of Send Logging (info on needed features and further info on how to utilize the entire feature set of send logging)