react-hook-fromで初期値を設定する方法を3つご紹介します。

defaultValues

1つ目はdefaultValuesを使う方法です。
この方法が一番一般的だと思います。

import { useEffect } from 'react';
import { useForm, SubmitHandler } from 'react-hook-form';

type Inputs = {
  name: string;
};


const Sample = () => {
  const {
    register,
    handleSubmit,
    formState: { errors },
  } = useForm<Inputs>({
    defaultValues: { name: '田中太郎' }
  });

  const onSubmit: SubmitHandler<Inputs> = (data) => {
    console.log(data);
  };

  return (
      <form onSubmit={handleSubmit(onSubmit)}>
          <input
            {...register('name', {
            required: true,
            })}
          />
        <button type="submit">Submit</button>
      </form>
  );
}

export default Sample;

setValue

import { useEffect } from 'react';
import { useForm, SubmitHandler } from 'react-hook-form';

type Inputs = {
  name: string;
};


const Sample = () => {
  const {
    register,
    handleSubmit,
    setValue,
    formState: { errors },
  } = useForm<Inputs>();

  const onSubmit: SubmitHandler<Inputs> = (data) => {
    console.log(data);
  };

  useEffect(() => {
    setValue('name', '田中太郎')
  }, [])

  return (
      <form onSubmit={handleSubmit(onSubmit)}>
          <input
            {...register(`name`, {
            required: true,
            })}
          />
        <button type="submit">Submit</button>
      </form>
  );
}

export default Sample;

reset

import { useEffect } from 'react';
import { useForm, SubmitHandler } from 'react-hook-form';

type Inputs = {
  name: string;
};


const Sample = () => {
  const {
    register,
    handleSubmit,
    reset,
    formState: { errors },
  } = useForm<Inputs>();

  const onSubmit: SubmitHandler<Inputs> = (data) => {
    console.log(data);
  };

  useEffect(() => {
    reset({ name: '田中太郎' })
  }, [])

  return (
      <form onSubmit={handleSubmit(onSubmit)}>
          <input
            {...register(`name`, {
            required: true,
            })}
          />
        <button type="submit">Submit</button>
      </form>
  );
}

export default Sample;

SWRで取得した値を初期値に設定する方法

SWRなどでAPIから取得した値を初期値に設定したい場合もあると思いますが、その方法です。

"use client";

import useSWR from "swr";
import { useForm, SubmitHandler } from "react-hook-form";

type Inputs = {
  id: number;
  name: string;
};

const Sample = () => {
  const fetcher = async (url: string): Promise<Inputs> => {
    const response = await fetch(url);
    return response.json();
  };

  const { data, error, isLoading } = useSWR<Inputs>(
    "http://localhost:3100/api/users/3",
    fetcher
  );

  const {
    register,
    handleSubmit,
    formState: { errors },
  } = useForm<Inputs>({
    defaultValues: {},
    values: data,
  });

  const onSubmit: SubmitHandler<Inputs> = (data) => {
    console.log(data);
  };

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      <input
        {...register("name", {
          required: true,
        })}
      />
      <button type="submit">Submit</button>
    </form>
  );
};

export default Sample;