The answer statement shows a dialog box with a string as message text:
answer <expression>
The following are examples for answer statements:
answer "Hello" answer x answer "The final amount is " & field "Amount"
The answer statement of HyperCard is much more flexible than this and can also serve to ask the user for a value, but this is not yet implemented in MoStacks: There is no possibility yet to ask the user for a value in an interactive way.