With the application focused, press Alt+F3 to bring up the window menu. In there, under "More Actions", you can uncheck "No Titlebar and Frame" for temporarily changing this.
But you're probably interested in permanently changing it. For that, select "Configure Special Application Settings..." instead.
In that window, click "Add Property..." and select "No Titlebar and Frame". Then change "Yes" to "No" in the newly added line and click Apply.
This changes it for that one application. You could also tell it to render a titlebar for all windows (with potentially unforeseen consequences) by changing the "Window class" dropdown to "Unimportant" and selecting "Normal Window" in the "Window types" dropdown.
If you change it like that, you can find this rule in the System Settings under "Window Management" → "Window Rules".