Sounds easy if the "ENV:" directory is available as well as ordinary CLI commands,
LAB main
CLEAR
ECHO "Enter a value: " NOLINE
ECHO >NIL: TO ENV:pvalue ?
IF VAL $pvalue EQ 1
ECHO "Starting program 1"
WAIT 2 SECS
SKIP main BACK
ENDIF
IF VAL $pvalue EQ 2
ECHO "Starting program 2"
WAIT 2 SECS
SKIP main BACK
ENDIF
IF VAL $pvalue EQ 999
ECHO "Quitting"
SKIP finish
ENDIF
ECHO "Incorrect value, try again."
WAIT 5 SECS
SKIP main BACK
LAB finish
Regards