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}\}