If you have OOP enabled, you can use the matrix forward vector multiplied by a scalar coefficient of the magnitude (i.e. strength of the velocity)
local magnitude = 0.5
local rocket = createProjectile(ped, 19, x, y, z, 1.0, nil, 0, 0, 0, ped.matrix.forward * magnitude)
Otherwise you can use
local m = getElementMatrix(ped)
local forward = Vector3(m[2][1], m[2][2], m[2][3]) -- offset from player position to position 1m ahead of them taking into account the rotation
local magnitude = 0.5
local rocket = createProjectile(ped, 19, x, y, z, 1.0, nil, 0, 0, 0, forward * magnitude)
Or, if you prevent to avoid using Vector3
local m = getElementMatrix(ped)
local forwardX, forwardY, forwardZ = m[2][1], m[2][2], m[2][3] -- offset from player position to position 1m ahead of them taking into account the rotation
local magnitude = 0.5
local rocket = createProjectile(ped, 19, x, y, z, 1.0, nil, 0, 0, 0, forwardX * magnitude, forwardY * magnitude, forwardZ * magnitude)