Each candidate for nomination for President of the United States and Vice President of the United States shall demonstrate the support of a majority of the delegates from each of eight (8) or more states, severally, prior to the presentation of the name of that candidate for nomination
That’s a good question, but the Presidential nominees select their running mates between the end of the nominating contests and the start of the convention.
For example, Mitt Romney announced Paul Ryan as his running mate on August 11th, 2012 and the nomination vote for President was on August 28th.
I assume the presumptive nominee gets his delegates to affirm they will vote for his pick of running mate, and turns that into the secretary.