Has anyone successfully implemented eTag as a header on a GET request?

We're trying to implement an eTag to reduce query time to a remote API.  They've provided support on their end and are generating an ETag,  a subsequent GET by us, with an ETag header doesn't seem to generate the response I'd hoped, and  before I go back to the vendor, I wondered if there was something tricky in the way the quotes need to be created around the ETag string?

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