You are correct the cost basis does not come into play for donated shares held long term. But the field box does require a number.
You can see if the mutual fund company can give you the average cost of the shares. Or you can make an educated guess and put some number in the field box to advance in the software.
The most important issue is to document that your donated mutual fund shares were long-term. Short-term shares would be limited to the cost basis for the donated value. As long as you can demonstrate that your shares were held for long-term, then your valuation amount should not be an issue.