See import command in the docs.

$ terraform import RESOURCE_TYPE.NAME ID
$ terraform import module.MODULE_NAME.RESOURCE_TYPE.NAME ID
$ terraform import 'RESOURCE_TYPE.NAME[0]' ID
$ # e.g.
$ terraform import 'datadog_synthetics_test.this[0]' ID
$ terraform import 'module.my_module[0].datadog_synthetics_test.this' ID