My 15 yr. old granddaughter and her custodial father (my son) have lived with me since 2018. Due to lack of income he will release his RTC to me. How fill this form ?

To clarify, what is RTC?

If you supply more than half their support (and meet the other requirements) you needn't have an official Right To Custody agreement if that is what you are asking. 

TurboTax asks the questions needed in the "Personal Info" section.

To qualify as a dependent, a child must also pass these tests:

• Relationship: Be your son, daughter, stepchild, eligible foster child, brother, sister, half-sister or -brother, stepbrother, stepsister, adopted child or the child of one of these
• Age: Be under age 19 or under 24 if a full-time student, or any age if permanently and totally disabled
• Residency: Live with you for more than half the year, with some exceptions
• Support: Get more than half their financial support from you
• Joint return: Not file as married filing jointly unless only to claim a refund of taxes paid or withheld

See the full rules for a qualifying child

Qualifying relative

A qualifying relative must meet general rules for dependents and pass these tests:

• Not a qualifying child: Isn't your qualifying child or the qualifying child of any other taxpayer
• Member of household or relationship: Lives with you all year as a member of your household or is a specific type of relative
• Gross income: Has gross income under $5,050
• Support: Gets more than half their financial support from you