Auto Generate primary ID using a custom prefix

You could roll your own solution to this. You could create a database table that has a column to hold the prefix e.g. 'APP' in your example, and then another column that holds the latest sequence number e.g. '01' in your example, and then write a Stored Procedure that takes those two values, increments the number part of it and writes that back to the same table, and then returns the concatenation of those two values. You can then call that Stored Procedure from Appian and use the returned value as your custom key for use where you intend to use it.

Stewart gave a good solution, but I'd also be curious how you plan to use this. Like do you actually need the value in the database or are you just hoping to have a friendly way of displaying the PK in Appian? If it's the latter, I'd recommend just storing the integer in the database and then have some method of formatting the result in Appian. For example, you could create a custom field that concatenates an app name in front of the identifer and it should work perfectly.

Keep in mind that you can expose other columns to end users and use the PK only "behind the scenes", There are pros and cons in DBs to not using integers as PKs. Relational DBs will implicitly build indexes on PKs to make lookups quicker and then whole numbers will generally be quicker and less resource consuming when querying by the PK.