noun
Refers to a physical opening designed for inserting something, such as a mail slot, coin slot, or ballot box opening.
郵便受けの差し入れ口から手紙を入れた。
I put the letter through the mail slot.
この機械の差し入れ口に硬貨を入れてください。
Please insert coins into the slot of this machine.
投入口 is a more general term for an insertion opening, often used for machines or containers, while 差し入れ口 emphasizes the action of sliding or inserting something in.
スロット is a common loanword for a narrow opening or slot, but 差し入れ口 is a native Japanese compound that can sound more descriptive or formal.
Compound of 差し入れ (sashiire, 'inserting') + 口 (kuchi, 'mouth, opening'). The exact historical derivation is uncertain, but the word transparently describes an opening for insertion.