I consider you want to set a random number for application number property. You can use @random function along with @round function to achieve this.
The random function returns a random decimal number between the given params (1000 - 2000) and round function converts the decimal to an integer.
Keep param values based on the number of digits you need for your application number.
Set this function return value to application number property using the property-set method in POST Activity or Set method in POST datatransform of flowaction if you want to set value after form submission.