On prem shared mailbox. May be one of my last posts about exchange being not in O365-land.

Open the Exchange Management Shell EMS

Create the mailbox
New-Mailbox -Name MAILBOXNAME -Alias ALIASNAME -OrganizationalUnit "OU path" -Database "DATABASE" -UserPrincipalName EMAILADDRESS -Shared

Give permissions to the mailbox
Add-MailboxPermission MAILBOXNAME -User "DOMAIN\USERNAME" -AccessRights FullAccess
Or
Add-ADPermission MAILBOXNAME -User "DOMAIN\USERNAME" -ExtendedRights Send-As

Convert a User mailbox to a shared mailbox
Set-Mailbox "" -Type shared
Add-MailboxPermission MAILBOXNAME -User "DOMAIN\USERNAME" -AccessRights FullAccess
Add-ADPermission MAILBOXNAME -User "DOMAIN\USERNAME" -ExtendedRights Send-As