How to add multipart boundary using setRequestProperty in Java?

Question

I saw here How to add parameter in httpurlconnection android the following :

            conn.setRequestProperty("Content-Type","multipart/form-data;boundary=" + boundary);

I didn't find at any place if I can split it and add the boundary by using setRequestProperty :

conn.setRequestProperty("Content-Type","multipart/form-data");
conn.setRequestProperty("Content-Type","boundary=" + boundary);

It didn't work, can you please advise?

Thanks!