Yes, you can report the rental fees from March to June as an expense at the end of the "vehicle" section. The "gain or loss basis" is the purchase price minus depreciation from the prior year you might have taken using the actual method. You will need to calculate 'Depreciation Equivalent' if you were using the standard mileage rate method.
The IRS provides a table with the depreciation equivalent rate in Publication 463, Travel, Gift, and Car Expenses. For example, if your business mileage is 1,000 miles in 2021, your depreciation equivalent will be equal to $260. (1,000 X 0.26 (depreciation rate per mile for 2021 from the IRS table)
