Hi Sebastien,
There are various ways that the confirmation can be customized.
Do you have a small number of lookup values?
The confirmation settings with conditional logic would probably be okay.
Do you have dozen(s) of lookup values?
You might want to explore whether the gform_confirmation filter would be a better way to programatically compare the field value to the list of values.
If the list of values will change over time, storing them as entries in a separate form (or DB table) may be more manageable than directly within the PHP code.
Do you want to do it with no/lower code?
I answered a somewhat related community forum post where the request was about looking up previously submitted unique ID values. Check out the video in it, that Gravity Flow’s Form Connector with its’ Update Fields step type could let you take a similar approach with no/lower code to solve your task. In your case, perhaps you add a URL field to your “user submitting form” that the Update an Entry field mapping defines the content of as the workflow executes. Then the confirmation URL could be set to be based on that URL field (or a conditional logic fallback if the url field is blank).
Cheers,
Jamie