Hi Alex,
You can simply run a check if the drawn geometry is contained by the boundary geometry using the contains or within method of GeometryEngine QML type. You need to make sure the geometries you are comparing are in the same spatial reference.
GeometryEngine QML Type | ArcGIS for Developers
If it returns false, then throw the message to re-input the geometry and clear the previously drawn geometry. You can also perform another check if the current location is within the polygon boundary. Otherwise you don't allow them to move forward with report such as disabling the Next button. It just depends on what is your requirement.
If you need help in how to get the geometry of the Polygon boundary layer, please feel free to open a support ticket to provide high level overview on how that can be done using ArcGIS Runtime.
Nakul