Anywhere within the file, format the deck that you want the notes to go into as follows:

{Target Deck Line}
{Deck name}

Or as:

{Target Deck Line}: {Deck name}

For example, with the default settings:

TARGET DECK
Mathematics

Or:

TARGET DECK: Mathematics

You may place more than one target deck in the same file, but only the first instance will be read and used.

See Folder settings for how to do this on a folder-level