More like this

serin2 portb.6,16468,[STR textline\8\13]

It will exit either filling 8 characters in the textline or earlier if you hit enter

If you want also to start filling textline after first enter then use this (not recommended though):

serin2 portb.6,16468,[WAIT(13),STR textline\8\13]

because serin may get confused which enter is, first or last. Better use different character for start and finish.

Ioannis