Your taxpayer doesn't control this.

A google search reveals Roth Ordering Rules: The IRS has prescribed a distribution hierarchy for Roth IRA assets. Contributions are always taken first; conversions (if any) are second in order by year of contribution, with converted pre-tax assets taken first and converted after-tax assets taken second. Earnings are considered distributed last.

Example:

https://www.investopedia.com/terms/o/orderingrules.asp

Assets are distributed from a Roth IRA in the following order:

1. IRA participant contributions
2. Taxable conversions
3. Non-taxable conversions
4. Earnings

...

There are also rules regarding specific assets. For example, contributions are distributed tax-free and penalty-free, and converted pre-tax assets are distributed without being taxed or penalized, providing that they have been held in the account for five years or more.

If the pre-tax assets have not been held in the account for at least five years, then a 10% fee would apply to the distribution. Converted after-tax assets, however, are always distributed tax-free and penalty-free.

Further earnings are distributed tax-free and penalty-free if the Roth IRA has existed for five years and the distribution is done on or after age 59 ½, or following death, disability, or a first-time home purchase.