A cloud-based identity and access management service for securing user authentication and resource access
This is a combination of the answers from Siva-kumar-selvaraj and DavidMoreno-4043. They get all the credit. I just wanted to put it in one screen since I don't always read all the comments and it took me a while to figure this out.
From Siva-kumar-selvaraj
Alternatively, you would have to use one of the "user.extensionattribute(s)" to map it to manager attribute by creating new outbound sync custom rule

Steps to create Inbound custom Rule on Sync server:
- Sign in to the server that is running Azure AD Connect sync by using an account that is a member of the ADSyncAdmins security group.
- Start Synchronization Rules Editor from the Start menu.
- Make sure Inbound is selected, and click Add New Rule.
- Give the rule a descriptive name, such as "In from AD – Send users SAM account value to Azure AD for salesforce". Select “local ad forest”, select User as the CS object type, and select Person as the MV object type. In Link Type, select Join. In Precedence, type a value that isn't currently used by another synchronization rule (for example 51 or 90), and then click Next.

- Leave the Scoping filter Join rules empty, and then click Next. (An empty filter indicates that the rule is to be applied to all objects.)
Adjustment from DavidMoreno-4043 in bold for step 6.
- Click Add Transformation, select the FlowType as Expression, and select extensionattribute(1-15) as the Target Attribute. In the Source text box, enter DNComponent(CRef([manager]),1). Click Add to save the rule.
More Details:
Safety precaution:
You can also try to enable staging mode and disable the scheduler on the Azure AD connect server while we are performing above mentioned steps and we will have to execute the full sync cycle except “export cycle of AAD connector” manually so that can make sure all changes are legitimate.
7.Post saving the sync rule. Open PowerShell wizard with Admin privileges and execute following command on the Sync server to Start-ADSyncSyncCycle -PolicyType Initial to trigger full synchronization. This step recalculates all attribute flows.
Hopefully putting their answers together will help someone else with this issue. I am thrilled to say this worked for me and I am very thankful for the info.
