There are 2 ways to connect your Page to a Business Manager account.
#1 Claim a Page
You claim a page that you own and add it to your Business Manager account. It is then manageable by everyone with access to your Business Manager, but no longer belongs to the Page owner. You must be both the Page owner and Business manager account owner to do this.
To claim one of your Pages, go to your Business Manager. Then go to Claim Assets, and then go to Page. Here all you have to do is copy and paste your Facebook Page’s URL, and then claim it.
#2 Request access
Ask the owner of a Page to add it to a Business Manager account. The Page owner will still own the page and will be able to choose who has access to it.
To request access to a Page, go to your Business Manager. Then go to Request Access, and the go to Page. Here all you have to do is copy and paste your Facebook Page’s URL, and select the role you want to have for this page. The owner will get the request and will decide your fate.