The code below should work to sequence the numbers in the format you want as long as it is for a single prefix and the sequence does not restart for different groups of records:
Parser: Python
Show Codeblock: checked
Pre-Logic Codeblock:
rec=0
def autoIncrement():
global rec
pStart = 10000001 # adjust start value, if req'd
pInterval = 1 # adjust interval value, if req'd
if (rec == 0):
rec = pStart
else:
rec = rec + pInterval
return "XTLF_XTLF{011d}".format(rec)
Expression (Label): autoIncrement()