Haven’t actually ran this BP but just as a mock up I think this is what you want.
Set up your input axis mappings like in the picture. The in your BP create 2 MakeVectors, one for horizontal movement and one for forward movement. Add these together and use it as your force. You’ll probably want to add a multiplier in there to get more control over the force amount.
Let me know if this works.
https://farm4.staticflickr.com/3728/13365575193_0a12b9d55a_o.png