This is easiest done via MP. Select Medic and try to find a Heavy to heal. Most people will target the Heavy so this should pop easily.
Alternatively:
Requires a Dummy account
Open the command console: ESC > Gear Icon > Under the keyboard tab select Advanced > Check off Enable Developer Console
Press the ` button to open the console and type:
map cp_dustbowl; hostname "boost"; sv_lan 0; sv_cheats 0; (I had to input this twice to be detected on my LAN for some reason)
That will create your match. Choose Medic on the Red Team. Then type the following command:
mp_teams_unbalance_limit 0; mp_idledealmethod 0; mp_idlemaxtime 0;
Set Dummy Account on Red and add 6 bots to Blue using
tf_bot_add 6 blue;
I recommend doing this at the 2nd Capture point. Put the Heavy on the 2nd Point. Put yourself somewhere close where you wont be shot at immediately and wait while healing.