Your form titled “prior year net contributions to Roth IRA” then, there’s a box to fill in an amount but it seems to be asking for all previous years contributions

[frmr-71]

So, do I just enter 2018 contributions? Or, do I go back to 2002 -2018?

[AnnetteB6]

You would enter the total contributions made since the Roth was established.  Be sure to subtract any previous year's distributions as well.  This number is establishing the basis of the Roth IRA.

[dmertz]

"You would enter the total contributions made since the Roth was established"

... except for contributions made for the current tax year.  Contributions made for the current tax year are entered under Deductions & Credits -> Retirement and Investments -> Traditional and Roth IRA Contributions.