The most easy and also most common way of getting access is ti simply contact the administrator for the organization you need access to. This person will then grant you membership and the access you require.
Applying yourself
If you want to apply for access yourself, you first and foremost need to be a member of the organization that owns the door / space. This organization must also have set the door as "public".
Go to "More" in the Accessy-app and then "Search" - if you can't find the access you're after here, it is likely that the organization has chosen not to let the door / space be publicly available for application;