Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Velocity Type
ID_FIELDFORCE_MODEParameter: Strength
ID_FIELDFORCE_STRENGTHParameter: Consider Mass
ID_FIELDFORCE_CONSIDER_MASSParameter: Field
ID_FIELDFORCE_FIELDLISTParameter: Display Box
ID_FIELDFORCE_DISPLAYParameter: Box Size
ID_FIELDFORCE_BOX_SIZEParameter: Line Density
ID_FIELDFORCE_DENSITYParameter: Display Vector Length
ID_FIELDFORCE_DISPLAY_LENGTHParameter: Length As
ID_FIELDFORCE_VECTOR_MODEParameter: Scale Displayed Length
ID_FIELDFORCE_SCALE_LENGTH_DISPLAY