No now for FY 2018-19 you cannot opt for composition scheme . As per sub Rule (3) of Rule 3, can opt for composition scheme by filing CMP 03 online on portal prior to the commencement of Financial Year .
Still if for next FY 2019-20: To opt for composition scheme a taxpayer has to file GST CMP-02 with the government. This can be done online by logging into the GST Portal.