How to add a space or comma(,) between two concatenating fields using mutate?

Hi Experts ,

Please help me to achieve as follow .I want something like
Fullname= vikas , gopal

With below mutate output is like vikasgopal. So I want to add , in-between first an last
mutate {add_field => {"Fullname" => "%{first}%{last}"}}


So... add the space or comma to the string?

mutate {add_field => {"Fullname" => "%{first} %{last}"}}

I tried it before posting this question , at that point it did not work but now it is working . Well thanks again Magnus