It's currently written in the Roman but I assume they used a different script before the Christian missionaries or whoever reached the area and spread English.
According to Language, 'Identity and Contemporary Society edited by Rajesh Kumar, Om Prakash', it seems like the earlier written form of Hmar was developed by Rev J H. Lorrain and Rev. F.W. Savidge to india, wherein the two arrived in Mizoram in January 11, 1894 and developed 25 alphabets modified from Roman alphabet to write the Mizo language. It would later be adapted for the Hmar languages. This is recorded as the first instance of written Hmar by G.A. Grierson in the Linguistic Survey of India (1900)