How to make a plaster cast on concrete?

Hey everyone, my mother passed away when I was younger. Recently I spoke to the current owner of the house I grew up in. They mentioned that my mother wrote our names on a concrete walkway. Id really like to make a plaster mold of the writing.

My question is, what would be the best way to do this? Im afraid pouring plaster directly onto concrete would result in it sticking since its porous. Any suggestions?