tnindie
Here is how I create a prismatic joint in the car physics with the Construct SDK(in this case, the left rear wheel):
var leftRearJointDef = new Box2D.Dynamics.Joints.b2PrismaticJointDef();
leftRearJointDef.Initialize(this.PhysBody, this.PhysLBWheel, this.PhysLBWheel.GetWorldCenter(), new Box2D.Common.Math.b2Vec2(1,0));
leftRearJointDef.enableLimit = true;
leftRearJointDef.lowerTranslation = leftRearJointDef.upperTranslation = 0;
leftRearJointDef.collideConnected = false;
this.LeftBackWheelJoint = this.PhysWorld.CreateJoint(leftRearJointDef);
If you need more information than that, I suggest you read the Box2d documentation and/or the Construct Physics files.