The ornamental design for a displayscreen with a user interface, as shown and described.
The FIGURE is a front view of a display screen with a user interface showing the new design.
The broken line showings of various elements of the user interface are included for the purpose of illustrating unclaimed portions of the display screen with a user interface and form no part of the claimed design. The perimeters of the portion of the underlying display screen and the user interface are understood to be flush.