If you want to raise your withholding rate, you need to ask your employer to do so. You are not able to do it yourself, and SKAT cannot do it for you either.
Alternatively, lower your deductions
You can lower your deductions (such as deduction for interest expenses or deduction for transport between home and work) in order to pay more tax during the year. In this way, you reduce the risk of having to pay outstanding tax the following year.
Be cautious if you enter a higher income
It is unlikely that raising your withholding rate by entering a higher income in your preliminary income assessment will have any effect. This method is only an option if you pay top-bracket tax.
If you enter a higher income and you earn less than DKK 342,860 in 2017 (DKK 332,920 in 2016), you risk paying too little tax. The reason is that if you enter a higher income, you will automatically be granted a higher employment allowance than what you are entitled to.