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Unfortunately, that's not possible at the moment.
Someone has created a feature suggestion to ask fitbit to add this and if you'd like to add your vote then you can find it here:
https://community.fitbit.com/t5/Feature-Suggestions/Add-a-timer-for-charge-2/idi-p/3212181
Note, though, fitbit's comment that they don't currently have any plans to make this change.
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