The box itself or the data already present in the field?
If you want the current contents of the field highlighted and selected when you first enter the field, select the field while in layout mode and then click the "select entire contents on entry" option found in the Behavior section of the Inspector's Data tab.
But if you want the field to highlight, you can change the background on all your 'enterable' fields to your highlight color. Then attach conditional format to them all at once with:
Formula = 1
... just type a 1 in the dialog after selecting 'formula' and set the background to white (or whatever color your NORMAL field background should be).
In this way, all the fields will evalute to true (1) and display white but when you click into a field, the conditional formatting disappears and the actual background (now actually the highlight) will show. The effect is that, whenever you click into a field, the entire field highlights (and it doesn't have to be set to select entire contents).
I set this up on all enterable fields on all enterable layouts where Users are working. It is easy to set up and uses minimal resources. :^)