This API lets you add multiple alation catalog objects to a domain, at once. If the count of requested objects is lesser than 1000, then it runs synchronously, otherwise asynchronously. The limit is configurable under property alation.domains.bulk_membership.sync_job_max_batch_size.
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curl --request POST \
--url https://alation_domain/integration/v2/domain/membership/ \
--header 'accept: application/json' \
--header 'content-type: application/json'