Go to the Velocity section in the modules, and click a small arrow next to it to the right. There under the User section you have to find Read From User Parameter or something like that, and when you click that, a variable will be added to the User section that’s in your screenshot.
Copy that variable’s name and then in a blueprint you can drag a pin from the referenced niagara system and type in “Set Vector3 variable”; in the node that you’ll get just paste the variable name and then you can set this parameter using any vector you want.
As for the Spawn Rate, it’s absolutely the same except you have to set not a vector3 variable, but a float variable for the Niagara System.