Populate Path Variables In Flows

I’m fetching a list of Stripe Checkout Sessions and then fetching those session’s Line Items in a flow. The latter request uses a path variable to specify the ID of the checkout session whose line items the request should fetch.

I’m storing the checkout session IDs in a durable, I can’t figure out how to populate that variable with that durable in a flow though? I’ve tried this but it doesn’t get picked up, with or without the colon -