You have two method to link the user active directory account to existing user account in Azure.
Soft match , is based on mail value or UPN. So have to ensure that you have the same value in Azure AD account and in the AD user account in local active directory.
Hard match , it based on MS-DSGuidConsystencyGUID attibut in on-premise user account and Immutable value in Azure user account. You should convert Immutable value and set it in MS-DSGuidConsystencyGUID attribut as mentioned in the link below:
Hybrid Identity: Getting Users Aligned
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