The short answer.
Not required. Don't mark your body with ashes or any mark. Repent of your sins daily not just for 40 days. You can't earn your way into heaven but only by grace.
What is Lent?
Lent is a period of about 40 days where Catholics participate in repentance and fasting. Usually they give up something near and dear to them. Something like no social media, video games or fasting for 40 days. The 40 days represent the the time Jesus spent in the wilderness, while being tempted by satan.
What is that black mark that goes on people's foreheads?
This black mark is from palm crosses from the last Palm Sunday which were burned. The ash from these palm crosses is used to mark the people with a cross on their foreheads to symbolise their repentance.
Is it ok for me to get that black mark on my forehead while fasting?
No. Jesus taught that fasting should be done discretely and you should anoint your head with oil and clean your face to appear fresh. Jesus said by doing this people would not be able to notice that you are fasting. A clean face clearly contradicts with rubbing ashes on your face.
"Moreover, when you fast, do not be like the hypocrites, with a sad countenance. For they disfigure their faces that they may appear to men to be fasting. Assuredly, I say to you, they have their reward. But you, when you fast, anoint your head and wash your face, so that you do not appear to men to be fasting, but to your Father who is in the secret place; and your Father who sees in secret will reward you openly." -Mathew 6:16-18
Should Christians observe lent?
Christians are free to do so. Repentance and obeying God's laws should be a daily routine not just heavily observed for 40 days out of the year and then treated lightly for the rest of the year.
What's wrong with Lent?
Lent should not be a time to boast or a time to gain God's blessing. Grace can not be earned. When Christ died on the cross the payment for your sins were paid in full! You can not earn/fast your way into heaven.
"For by grace you have been saved through faith, and that not of yourselves; it is the gift of God, not of works, lest anyone should boast" - Ephesians 2:8-9
Don't use the 40 days of lent to focus and obey God's laws. Practice and obey God's word daily. Also, you don't need a mark on your face to prove anything! Focus on pleasing the heavenly Father not men.