Use
$LTI{v1p3}{preferred_source_of_username} = 'https://purl.imsglobal.org/spec/lti/claim/lis#person_sourcedid';
This is documented in conf/authen_LTI_1_3.conf.
WeBWorK Main Forum
How to see LTI 1.3 debug info?
This forum has a limit to the number of forum postings you can make in a given time period - this is currently set at 10 posting(s) in 1 day