If I understand you well you have a form with dropdown and you want the value of the dropdown to filter
the query given by the eventtype. if I'm not wrong, What you have to do is:
1- create your dropdown populated by a search that gives you the appropriat values you will use to filter the eventtype
2- place the token in the query as you desire.
THE FOLLOWING IS AN EXAMPLE OF FORM LIKE THAT(i'm using splunk 6.2):
the eventtype is about source and i filter it with sources values from the dropdown.
<input type="dropdown" token="source">
<query>index=* source=* |stats c by source</query>
<query>index=* eventtype=source_eventtype|search source=$source$</query>