Where is the option on Ebay to put in a maximum bid?

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I previously bid on an item and the option to either put in a maximum bid or a one-time bid. Somehow, when I bid on an item now, this option does not come up anymore, there is only a field to enter the amount, not the option to put in a maximum bid. How can I get to the maximum bid field?

Additional Details

well i seem to get two different scenarios: one is where you can either press a button that sais: "bid $10.11" (for example) or enter maximum amount, the other one i get is that you can only confirm the bid you put in initially.

so if ebay always only puts in the lowest amount necessary, then how come that in auctions the price sometimes changes from let's say $5 to $20 without any bids in between?

Where is the option on Ebay to put in a maximum bid?