To show specific coupons with a shortcode, you have to get the IDs of the coupons you want to show. Here’s an example of the shortcode:
Coupons with corresponding IDs will be shown where the shortcode is put.
Now to get the IDs of coupons, check your Coupons list. You will see a column with the IDs of the coupons.
Copy the IDs from here and paste them on the shortcode. Make sure there’s no space between the IDs in the shortcode, only use commas to separate them.