Heat transfer equation in PDE Comsol

In the heat transfer equation as attached file, it exits a part: rho x cb x omegab x (T-Tb)

However, in Coeffient form PDE equation, only has part aT. Then a = rho x cb x omegab Could you help me to do the part: rho x cb x omegab xTb? It can become f in PDE equation. Thanks