If the difference between the two base searches is more than just one value (sourcetype) you could create two panels - Windows / Linux and show/hide them based on user selection in the dropdown. Something like this may work...
<row>
<panel>
<input type="dropdown" token="selectedOS" searchWhenChanged="true">
<label>Service Provider</label>
<choice value="Windows">Windows</choice>
<choice value="Linux">Linux</choice>
<change><condition value="Windows"><set token="BS_Windows">Windows</set><unset token="BS_Linux" /></condition>
<condition value="Linux"><set token="BS_Linux">Linux</set><unset token="BS_Windows" /></condition>
</input>
<panel depends="$BS_Windows$">
<title>Windows Compliance</title> --> Make sure you add an `eval temp="$BS_Windows$"` to your query to prevent the execution if token not set.
<query>VISUALIZATION</query>
</search>
</panel>
<panel depends="$BS_Linux$">
<title>Windows Compliance</title> --> Make sure you add an `eval temp="$BS_Linux$"` to your query to prevent the execution if token not set.
<query>VISUALIZATION</query>
</search>
</panel>
</row>
If it is only one value, you can use token in the base search itself. `index=xyz sourcetype="$selectedOS$"
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