Hello @caitlin.benn ,
It would have to depend on how these subscribers are coming into your segments. It would most likely have to be a custom made solution to attach a metric/property that could be tagged to those subscribing via Bold.
If you want to send Bold data into Klaviyo using Bold's webhooks, Klaviyo's support team is well equipped to help with using data already within Klaviyo. Unfortunately, they are not the best resource for getting data into Klaviyo in the first place. That is something that requires developer knowledge from the source. If you need help setting up this connection I recommend reaching out to a developer. You can reference our approved partners list here.
https://klaviyo.partnerpage.io/
While we are not able to help with building custom solutions, we are able to help if you run into any issues. All of our API documents can be found here. Let us know if you have questions about those.
https://apidocs.klaviyo.com/reference/api-overview
Thanks,
Alex