I have a 4'' concrete curb poured on top of a concrete slab .Water flows under the curb because it wan't keyed or bonded to the slab.
Can I use one of the urethane sealants at the joint.The previous owner tried first a sloping concrete overlay to push the water away from the joint,then plastic cement, then a bead of silicone on the edge of the taper. All failed
I thought of using hydraulic cement but would not be able to push much between the joint between the 2 surfaces although there is a v-notch where the 2 surfaces meet

I think you're right. Urethane is the way to go. If there is any room for foam backing rod, use it but it doesn't sound to be the case. Be meticulous about cleaning the joint prior to sealant application. If it's a very important location, it might be worthwhile to grind, chip or cut out a notch at the joint to get more contact and better adhesion.

I think that spring is right on with the backer rod suggestion even if it is necessary to make room for it. They can often be forced back into a fairly small opening.
If you can make that v-notch wider in the back than in the front your sealant will be more likely to stay in there. In other words, it would still be a vee more or less but the point of the vee would be to the front instead of to back the back.