i would sew it onto a piece of live rock with a neddle and fishing line.
they will be a bit fussy after being cut off the glass and may take a week or more before it starts to expand. question - did they just cut it and give it to you or did they cut it and then leave it in their tank for a while to heal before selling. i ask cause moving fresh cut corals is not a good practice they are stressed enough from the cut and then now it has to also acclimate to a new system. it may still be OK if that is the case but next time........ let it heal first