Hi,
I believe there is no endpoint to find user accounts that are defined as manager of other accounts.
There are many different ways possible of course, this is an example :
One of the solutions that can meet your needs (it depends on how many user accounts to request) is to retrieve the list of all your users. Then, for each user, use the Get-MgUserManager command and retrieve the information as follows:
# Once you get all you users, for each :
Get-MgUserManager -UserId $UserId
$managerUPN = $managerInfo.AdditionalProperties['userPrincipalName']
$managerDN = $managerInfo.AdditionalProperties['displayName']
#And then store into an array or export in CSV directly