The + button is disabled in the Preferences > Talend > Run/Debug > Remote window, and you cannot add a Remote JobServer to run Jobs from Studio to the Target Exec JobServer.
When Studio is logged into a remote project (using a remote TAC connection), you cannot add a Remote JobServer from Studio because it's controlled by TAC.
Studio won't display any Remote JobServers because there aren't any JobServers listed on the Servers page in TAC.
In TAC, add the JobServer to the Servers page.
You are now able to see the same JobServer listed in Studio, and you can use it to run the Job using Target Exec.