Hi,
Do we have model popup (example in below image) in Appian like we have in HTML or any other language, If Yes, how can we implement it on Interface, If No, what's the alternative for it in Appian (some sort of side panel or something which fulfil my requirement of popup)?
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The answer used to be a simple "no", however we now have the ability to make related actions / record actions open in "dialog" which essentially creates a (full-page size) pop-up. It's not very configurable (like, it'll always show up with an "X" in the upper corner that the user can use to effectively cancel out of it even if you want to force them to use on-form buttons to cancel), but it might satisfy your use case.