creating records. How customize before saving

In my form at the front, there are 2 input fields: first_name and last_name.
But in the User model (and DB) at the back, there is 1 field: full_name.
Should I use mutateFormDataBeforeCreate(array $data): array or beforeCreate(): void? ( https://filamentphp.com/docs/3.x/panels/resources/creating-records#customizing-data-before-saving )
If it's beforeCreate, then how can I pick up the submitted form data? (the documentation didnt illustrate that)
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