How I did was to go into the club manager that is listed in the scroll below my avatar. I looked down the list to find the arena adding and it says to add the numbers of the arena there.

When you go on your arena page, click the link at the top of the page and right at the end is the four numbers for the arena. Go back to the club manager and add the numbers from the link into the space and click submit.

Examples:

https://i.imgur.com/diXpH6c.png

(Club Manager Arena adder.)

https://imgur.com/vqmfsL2.png

(For the last for numbers for the arena. By the way this is a example so yours won't have these same numbers.)