Updating Global Variables

I am trying to use a global variable to update row numbers in a repeating subform. I defined a variable 'rowCount' where the value is "1" from the Form Properties > Variables menu. I would like that value to go up by 1 whenever a user clicks something like an 'add row' button. I tried the following, and searched around but can't seem to find the answer:

rowCount.value = rowCount.value + 1;