I have earned income from foreign country in 2018 and paid tax to the country where the money earned but not qualified for exclusions. how could I apply for tax credit?

If you have not filed, here are the steps (online):

• Sign in to your account, select Pick up where you left off
• At the right upper corner, in the search box, type in foreign tax credit and Enter 
• Select Jump to foreign tax credit 
• Start with screen Foreign Taxes, select Continue  
• Next screen, Do you want the Deduction or Credit? select Take a Credit
• Follow prompts
• On the screen, Foreign Taxes Paid, enter your amount.