I already found: https://confluence.atlassian.com/jirakb/creating-issues-via-direct-html-links-159474.html
resulting in: /secure/CreateIssue.jspa?pid=11401&issuetype=10504&customfield_17002=product1
And know how to prefill project, issuetype, jira fields. ....
Also custom fields work.
But how to pre select the value/option of a custom field select field
&customfield_17002=product1
OR
&customfield_17002=20907
does not do the trick.
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