You cannot specify talkgroups & slots when programming as a conventional channel.
Instead, simply create a One Frequency Trunked system. The single frequency is used as the site. You also create a Department to contain your various talkgroups. You are not required to specify a slot, but there are a few systems that I've seen where the same TGID is used for two different uses, such as Slot 1 for the PD, and Slot 2 for the FD.
If it is system where you know there are several talkgroups involved, as '
shades has noted, you will need to listen to determine which TGID is used for what purpose. Using logging in ProScan can help clarify how many different users are active.