While a VERY picky rules lawyer could maybe argue that it isn't explicitly said so, we can reliably infer that tokens are purchased only after the company has floated (which requires 50% of shares be purchased). My reasoning for this is that the rules state that shares purchased from the IPO are paid to the bank. There is no disclaimer about non-chartered companies here because once the non-chartered corp floats, its certs move from the IPO to its charter and then it is funded for the shares sold thus far. It would therefore be impossible to pay for the tokens prior to 50% of shares being sold.
It is actually explicit.
6.5.3 Starting a new company -
3. [...]The player takes the company charter, places the rest of the company certificates on it, and then pays the company three times the stock-market price for the director's share (not the bank).
The next section -
Flotation occurs when 50% of a company's shares have been bought from the IPO or company.
188.8.131.52 Station Markers
[...]These are to be paid for immediately...
The purchase of Station Markers is contained within the rules for company Flotation and not Starting a new company. So tokens are purchased immediately upon flotation.