Use a wait stage with two branches. The first branch condition is the happy path, where the credentials are valid; the second branch is the unhappy path where the pop-up appears. Obviously if the credentials are invalid there probably isn't much your process can do to fix that, and chances are you'll just have to abort. But if you want to handle the pop-up and maybe issue a notification to the sys admin, then modelling the unhappy path is the way to go.
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John Carter
Professional Services
Blue Prism
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