Apologies, you seem to have misunderstood. Moodle is complaining of "Empty sourceid". I am asking how Moodle populates sourceid.
These are the values in my request:
name= ext_basiclti_submit value= Something
name= ext_user_username value= user
name= launch_presentation_document_target value= iframe
name= launch_presentation_return_url value= foo.url
name= lis_outcome_service_url value= https://eoljidknqpe0mkg.m.pipedream.net
name= lis_person_contact_email_primary value= user2113@example.com
name= lis_person_name_family value= Darwin
name= lis_person_name_given value= Charles
name= lis_person_sourcedid value= 1123511
name= lti_message_type value= basic-lti-launch-request
name= lti_version value= LTI-1p0
name= oauth_callback value= about:blank
name= oauth_consumer_key value= somevalue123
name= oauth_nonce value= 3924215557000
name= oauth_signature value= l9BF5Y935VlIyG5v2m+dleOIN18=
name= oauth_signature_method value= HMAC-SHA1
name= oauth_timestamp value= 1681915848
name= oauth_version value= 1.0
name= resource_link_id value= 1
name= roles value= Student
name= tool_consumer_instance_description value= description
name= tool_consumer_instance_guid value= put your site url here
name= tool_consumer_instance_name value= My glorious platform
name= user_id value= 1123511
