For the moment on SportEasy, it is not possible to associate a stadium with an opponent permanently. This is a feature we will be adding as soon as possible.
For now, when the coach and/or team administrator creates an event for a match, he/she must edit the stadium where the match will take place themselves. It will not automatically appear with the name of the opponent even if it was previously created in the opponents list (see photo below) of your team. (See "Create an opponent" for more information).
Thank you for your patience!