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What exactly are you looking for? Is that you are trying to make a existing field to be required for subtask issue type under a particular project?
You are trying to create a new custom field and make it required?
making a custom field required in the field configuration will make it required for every other projects in which you make use of the field which you have created and by doing this your question stating of making it required for a subtask for a particular project would not be achieved.
you can make a field mandatory in two ways
1. you can make mandatory on field configuration, go to the field configuration which you configured for that issue type on which you want to make mandatory then search for component field and click on required option
check here for more information(Making a field required or optional)
another one is by configuring field required validator on required transition
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