Tracking Opt-In Source (LP Name)

If an existing lead in our database checks an Opt-In box on a form (setting Opt-In to True), how can I populate an Opt-In Source field with the name of the LP?

I can see it in the Reason field in the "Change Data Value" activity in the activity log but cannot figure out who to pull this information out in, say, a token.

I.e., here's what I have in mind:

Smart List Trigger: Data Value Changes->Opt-In

Contraints:

- New Value->True

- Source->Web form fillout

Flow:

Change Data Value->Attribute->Opt-In Reason

New Value = {{Token that Captures the info in that Change Data Value Reason Field}}

Is there some way to do that?