Try doing an eval in your search proper:
<search>
<query>... | eval endpoint="/$service$/$environment$" | ...
Or try doing an <eval>...</eval> on change from both dropdowns:
<input type="dropdown" token="service" searchWhenChanged="true">
<label>service</label>
<choice value="capi">capi</choice>
<choice value="crapi">crapi</choice>
<choice value="oapi">oapi</choice>
<default>capi</default>
<initialValue>capi</initialValue>
<change>
<eval token="endpoint">"/"+$service$+"/"+$environment$</eval>
</change>
</input>
<input type="dropdown" token="environment" searchWhenChanged="true">
<label>Environment</label>
<choice value="prod">prod</choice>
<choice value="ppe">ppe</choice>
<choice value="pte">pte</choice>
<choice value="dev">dev</choice>
<default>prod</default>
<initialValue>prod</initialValue>
<change>
<eval token="endpoint">"/"+$service$+"/"+$environment$</eval>
</change>
</input>
from User warren – Stack Overflow https://stackoverflow.com/questions/62588370/how-do-i-set-a-token-based-on-the-dropdown-options-in-splunk-dashboard/62594826#62594826
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