https://github.com/jenkinsci/emailext-template-plugin/blob/87963f42ab52ee2f64dac1a2cf61ca9be8a76357/src/main/resources/org/jenkinsci/plugins/emailext_template/ExtEmailTemplateManagement/index.groovy#L63-L69
Also the same file has an onclick handler
https://www.jenkins.io/doc/developer/security/csp/
jenkinsci/emailext-template-plugin/pull/128