To answer my own question; found it in the API log:
/api/v1/add/alias-domain
{“active”:[“0”,“1”],“alias_domain”:“alias.tld”,“target_domain”:“target.tld”,“rl_value”:"",“rl_frame”:“s”,“dkim_selector”:“dkim”,“key_size”:“2048”}
Domain alias list:
/api/v1/get/alias-domain/all