The ornamental design for a displayscreen or portion thereof having a graphicaluserinterface for a link to a datainterface, as shown and described.
The sole FIGURE is a front view of a display screen or portion thereof having a graphical user interface for a link to a data interface showing our new design.
The broken lines depict portions of the display screen or portion thereof having a graphical user interface for a link to a data interface in which the design is embodied that form no part of the claimed design.