Hi @Quentin Baker ,
Thank you for posting query in Microsoft Q&A Platform.
As per my understanding, requirement is more related to using parameterized linked service in External call transformation. Feel free to correct if its my the case.
As mentioned in above comment, I reproduced this issue and found that external call transformation not allowing to pass values to linked service parameters. I reached internal team to confirm same. Its confirm that at this moment external call transformation not supports linked service with parameters. PG will be considering this to implement and rollout.
I will be updating thread once feature gets develop and releases.
Hope this helps. Please let us know if any further queries.
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