You can give transient velocity to the moving wall using a UDF. The UDF updates the wall speed internally according to user specified time variation. You need to specify the wall as "moving wall" in the boundary condition wall. Specify how you want to give the velocity (Translational, Rotational, or components). Specify whether the values are absolute or relative to adjacent cell zone. If you enable these options in GUI correctly, then you can vary the values using UDF.
The UDF uses a macro which directly assigns the velocity to the thread corresponding to the moving wall, instead of going to the GUI to use the value assigned by user. So this UDF will not update the changes in the boundary condition panel corresponding to moving wall. To see the updated values on the moving wall, one has to display contours of velocity on that wall with node values enable and global values disabled. The Min and Max values reported in the Contours panel will be the updated values of the velocity assigned to the moving wall.
See the attached UDF file.