You've created a password using the combination screen in Mecrets but now you want to enter it using the keyboard. You can absolutely do that but you need to separate them by a comma and a space.

e.g. if your number is 23 followed by 12 followed by 98, enter 23, 12 , 98

Also, if you are on the keyboard screen, you can always get back to the Combination screen by tapping "Combination" in the upper right corner.