[Python] How can I pass Jinja2 templated parameter to a task using the Taskflow API?

I have a task that is not correctly rendering one of its parameters if I pass it as "{{ var.json.my_api_key.api_key }}":

@task
def generate_signature_headers(api_key="{{ var.json.knmi_api_key.api_key }}",
hmac_secret_key="{{ var.json.knmi_api_key.hmac_secret_key }}".encode('utf-8')):

hmac_digest_transformed = ...some code..

return {
"Date": now_utc,
"Authorization": f'Signature keyId="{api_key}",algorithm="hmac-sha512",'
f'signature="{hmac_digest_transformed}" ',
}


the api_key parameter is returned as the string "{{ var.json.knmi_api_key.api_key }}" without being rendered, but hmac_secret_key works as expected.
Everything works if I pass the api_key parameter as Variable.get('knmi_api_key', deserialize_json=True)['api_key'] but doing so, it is going to create a connection to the metastore database every time the dag is parsed.

How can I pass the parameter with the Jinja2 templating engine?

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