FCSetScalarProducts[] assigns values in the second list to scalar products (or other kinematic-related symbols such as Momentum, CartesianMomentum, TC etc.) in the first list.
The values can be also modified if the quantities in the first list are entered by hand. To modify the definitions programmatically without resorting to With and similar delayed evaluation tricks one can use placeholders in conjunction with the InitialSubstitutions option.
FCClearScalarProducts[];
FCSetScalarProducts[{SPD[p1], SPD[p2], SPD[p3, p4]}, {0, xx1, xx2}];{SPD[p1], SPD[p2], SPD[p3, p4]}\{0,\text{xx1},\text{xx2}\}
FCSetScalarProducts[{spd[p1]}, {val}, InitialSubstitutions -> {spd -> SPD}]\{\text{val}\}