If the employer contributes only $125 for the year, the remaining $458.33 that can be contributed without using the last-month rule can be contributed by you through payroll deduction or by personal contribution. If the contributions through your employer are through a cafeteria plan, those contributions are be exempt from Social Security and Medicare taxes,but if you make a personal contribution you will have ended up paying about $35 in Social Security and Medicare taxes on the $458.33 (assuming that you will have not maxed out on the Social Security wage base for the year).
2018 TurboTax uses the self-only coverage limit of $3,450, not the 2019 limit of $3,500, so without using the CD/download version of TurboTax and using overrides you cannot get an accurate calculation from 2018 TurboTax. You'll need to wait for 2019 TurboTax to get a correct calculation without using overrides, and even the initial release of 2019 TurboTax might not have the necessary updates; cost-of-living-dependent limits are often not updated until a few weeks after the initial release for a particular year.
