You could use a text input to modify a multiselect dropdown - when the text is less than 3 characters set the filter to head 1 (to hopefully speed up the query), otherwise set it to an appropriate where clause. <input type="text" token="keyprefix">
<label>Select a key prefix</label>
<change>
<eval token="prefix_choice">if(isnull('form.keyprefix') OR len('form.keyprefix')<3,"head 1","where like(key,\"".'form.keyprefix'."%\")")</eval>
</change>
</input>
<input type="multiselect" token="key" searchWhenChanged="true">
<label>Select a key</label>
<search>
<query>query | $prefix_choice$ | dedup key | fields key</query>
</search>
<fieldForLabel>key</fieldForLabel>
<fieldForValue>key</fieldForValue>
<prefix>(</prefix>
<valuePrefix>key ="</valuePrefix>
<valueSuffix>"</valueSuffix>
<delimiter> OR </delimiter>
<suffix>)</suffix>
</input>
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