If I understand you correctly, you want to have the drilldown target be an external link.
If that is correct, just put the external URL in the drilldown: 
If you want some value from an external link to be pulled-into Splunk, then you’ll need to setup another mechanism.
For example, you might have a scripted input that will query an external endpoint, and add results to an index or lookup table.
Or you might utilize a REST endpoint app to achieve something similar.
from User warren – Stack Overflow https://stackoverflow.com/questions/64963690/is-it-possible-to-call-a-url-in-a-splunk-dashboard-and-get-the-response-as-a-str/64971025#64971025
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