To associate a stadium with your home matches, you must first have created your event (Championship, Cup, Friendly,...).
Then go to the tab "Calendar" > "Settings" >> "The name of your event".
In the General Information, you will see "The Default Stadium". If you had created it before, just click on the arrow at the bottom and you will find it, otherwise, click on "Create a stadium". You will then be brought to the Stadium Creation feature (see the section Create a Stadium or a gymnasium, an ice rink, etc.) for more help.
You can then choose to "Apply the stadium to all your home matches" (if you are playing in the Championship for example) or "Apply to all matches" of your season (for example if you play matches with friends in one particular place all year round).