Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Velocity Type
ID_FIELDFORCE_MODE
Parameter: Strength
ID_FIELDFORCE_STRENGTH
Parameter: Consider Mass
ID_FIELDFORCE_CONSIDER_MASS
Parameter: Field
ID_FIELDFORCE_FIELDLIST
Parameter: Display Box
ID_FIELDFORCE_DISPLAY
Parameter: Box Size
ID_FIELDFORCE_BOX_SIZE
Parameter: Line Density
ID_FIELDFORCE_DENSITY
Parameter: Display Vector Length
ID_FIELDFORCE_DISPLAY_LENGTH
Parameter: Length As
ID_FIELDFORCE_VECTOR_MODE
Parameter: Scale Displayed Length
ID_FIELDFORCE_SCALE_LENGTH_DISPLAY