Create a copy of "inputdlg.m" e.g. as "inputdlg2.m". Search for "uicontrol(EditHandle(1))". Add this line after the following END:
Now the OK button is selected initially and hitting ENTER activates it. If the inputdlg2 files is created in a user folder (recommended!) insert the files MATLABROOT\toolbox\matlab\uitools\private\getnicedialoglocation.m and \private\setdefaultbutton.m also in inputdlg2.m.
Most likely a completely new created dialog file would be much smaller.