There is a Tax Form thus there's probably a reason for such a form.
Here's a list of companies that pay between $1000 and $100K. In addition to asking the employer for bonuses or reimbursement you can ask the school about other means to obtain money.
Read "What are scholarships, grants and bursaries?".
Scholarships are money awards you get for your academic performance (high grades) or other achievements. Grants are money awards you get based on high grades or financial need. You can also receive a grant when you carry out academic research projects. Bursaries are money awards you get based on your financial needs.
You can ask during the interview but paying for a PhD is likely to be seen as an unexpected and expensive request; expect it to affect bonuses and pay. The chances of them paying, you going to school, and then you returning would be a gamble for the company.
I know people who have asked people with whom they are acquainted and own a business if they paid for a course that the business owner suggested would they then hire them for the position if they came back with great Marks. The business owner had lots of money and little to lose, despite that they declined.