You can't do this. The whole config of an issue hangs off project and issue type.
The create process needs to know which issue type you want to create so that it can give you the right screen for the issue.
Later versions of JIRA do allow you to dynamically change this, so you go into create and changes the fields when you change the selected issue type. This is not what you're asking for, but it does solve the problem.
In Jira 4 and below, the create issue button went to a screen that said "tell me project and issue type", then you got the create screen for that combination. In 5 or 6, it changed to a system which gave you those two at the top and if you change them, the fields below change to the screen represented by the new combination.
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