Step.Requirements
Returns the Requirements property for the step. The Links subproperty of the Requirements property is an array of string values that represents the product and unit requirements the step covers.
You can use the following pseudo code to add a new element to the list of requirements:
PropertyObject links = step.Requirements.GetPropertyObject("Links", 0);
int nextAvailableIndex = links.GetNumElements();
links.SetValStringByOffset(nextAvailableIndex, PropertyOptions.PropOption_InsertElement, "REQ_ABC");