getVisitStart function or Page = 1 for Visit first page?

It can be done by checking and setting a cookie in two steps

For example:

Step1: Check for a cookie "firstVisit"(you can assign any random name) when a visitor lands on the first page.

the DTM rule will be triggered when it does not found the cookie(you can use custom code)

Step2: Create another tag on window load which sets the cookie which you were searching for in the first tag and was not set, this will ensure that the first tag doesn't fire on the next pages or reload.

Make sure step 2 triggers only after step1 is triggered