Hi @Susan Dodds welcome to Azure! To sync your on-premises Active Directory with Entra ID, you will need to set up Entra Connect. Entra Connect is a tool that allows you to synchronize your on-premises Active Directory with Entra ID, which is the identity provider for Office 365.
Since you don't have an Entra ID subscription, you can sign up for a free trial.
Once you have set up Entra Connect, your on-premises Active Directory users will be automatically synchronized with Entra ID. This will allow your users to access Office 365 using their Entra ID credentials.
Please let me know if you have any questions and I can help you further.
If this answer helps you please mark "Accept Answer" so other users can reference it.
Thank you,
James