Not sure how to do this using the graphical user interface as it was, but in xmlyou can set it like this:
<TRANSITIONS>
<TRANSITION from="" to="New">
<REASONS>
<DEFAULTREASON value="Identified" />
</REASONS>
<FIELDS>
<FIELD refname="System.AssignedTo">
<DEFAULT from="currentuser" />
</FIELD>
</FIELDS>
</TRANSITION>
.....
</TRANSITIONS>
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