Although it is likely possible to do this with rules, it will be complicated. Best bet is to use code.

I have a command button.  After the user clicks on it I want to populate a
text box with a "CR" and the current date/time formated as mmyyhhmm.  For
example, the resulting string would be CR01054505.  Please help.  Thanks!

The now() function returns the current system date and time in a format like
yyyy-mm-ddThh:mm:ss. So you can use this information to create a rule on the
button that sets the value of the text box using a formula like:

concat("CR", substring(now(), 6, 2), substring(now(), 3, 2),
substring(now(), 12, 2), substring(now(), 15, 2))

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S.Y.M. Wong-A-Ton