How to create custom field in posts?

Hello forum! My site is https://www.e-win.gr and i run Habitat theme.

As you see in every post there is under the title a bubble icon for the comments , an archive icon for the categories and a calendar icon for the post publishing date!

My site has to do with competitions with presents and coupons so I want to add a last icon that will show an expiration date of the competition or the coupon. In every post the date may be different and i don’t know how to do it!

An example for what i want to do is: [ICON] “Expiration Date: (the date is different in every post”

The text “Expiration Date” is permanent and the date must be varies.

How can i do this? Please help me because is so important!

Thanks in advance

P.S. Sorry for my English.