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You can ask them to process an amount more than the bill itself or, more normally, leave a tip in cash. In cash is more likely to find its way to the people serving you, rather than management. Often also no tip is expected because a service fee has been added anyway. Cash may also be appreciated as a means to evade some tax.
Sometimes the bill does have a place for a tip and is left for a little while with the customer to review and possibly find the right amount of cash. It is only on a later visit that either cash or card is collected.