Thank you, I got it working! Now it gives nice boost from jumping slope!
So for anyone with similar problem later, I did following:
I created empty object, put Raycast 2D on slope. When slope Raycast is on player, it’ll next detect if player is sliding, after that, detection for a jump. If so, Add Force 2D action (my way is to put for X 0,1, Repeat Every Frame, and then Wait action for about 1 second before finish).