Can I reference a Custom Object in a Segmentation List?

[SanfordWhiteman]

then the Webhook is the easiest solution for her to do this?

Not using a CO at all -- running a webhook against a CSV or just a text {{my.token}} -- is the easiest way.  A standalone CO is a square peg here.

If so, then where does the "Schedule Campaign API call" get triggered from - within Marketo?

No, it would have to be triggered by whatever external process has firsthand knowledge that there's been a change/add to the CO.

[Anonymous]

So the easiest solution is to create a web hook and in it run the check of the domain values against the lead emails, setting a flag on leads where the condition is met. Is that correct? And does Marketo provide the ability to run web hooks on a schedule? Thanks.