Sending to a Source ultimately ends up in a Topic, so if you would like to send to a Topic, send to the Source connected to that Topic.
Begin at Studio Post Manager.
To send to a Source, go to the [Review] screen on the Post you want to share. Once you are there, select [Add Contacts]. This will bring a pop-up menu that will say "Send to Devices." At the top right of that pop-up menu, select the dropdown menu that says Your Company Name - Members, Sources or Teams. Then choose "Your Company Name Sources."
Source names are displayed alphabetically and the "Find Contact" can be used to search for a Source name. Select which Source you want to send the content to by clicking on the name (notice the radial button shows a check mark). Clicking the name again, deselects it. After your selection, click [Close]. Remember, sending to a Source will display your Post under the Topic in which it is connected. Lastly, you will select the purple send button (bottom right corner of the screen) to send your Post.