GUI help with inserting variables

I have created a GUI where the user inputs a speed into a 'editbox' which is followed by a 'calculate' pushbutton. the calculate push button when clicked runs an external function and it will display warning messages as appropriate.my variable is called 'h' and it is a part of my external function. this is the variable that will be input by the user in my editbox. How do I assign matlab to use the value entered in the editbox and use it in my external function