Head to the Configure tab:
Click on the 'Edit Simulcast' button (small pencil icon) next to the show artwork on the left-hand side.
Then, click on the dropdown menu and select the relevant matching YouTube show, then you should be able to manually match YouTube episodes. After that, click on Save.
After refreshing the page, the show should be updated as a simulcast with episodes having two versions - an RSS version and a YouTube version:
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