Bank Accounts tab if you have not already. Create the fixed deposit as a new bank account under the
Bank Accounts tab. Manager will automatically classify the account as an asset. Record the money you place into the fixed deposit account using an appropriate method. Probably, you will be transferring money from another account you already have, but you might be depositing a new contribution of capital.
Make sure you have created some type of interest income account. This might be a general account for all interest or you could create a specific one for the fixed deposit. As interest is earned, record it appropriately. If the interest is credited by the bank to the fixed deposit, use the Receive Money function under that account and credit interest income. If it is paid separately, you will need to Receive Money in whatever bank account you allocate the interest to, still crediting interest income. How often you do this will depend on how often you receive statements or the bank pays interest. If you receive no statements or interim payments, good accounting practice should reflect the interest you have accrued and could withdraw. But if the terms of the fixed deposit will not allow withdrawal of interest until the end of the term, you might legitimately delay accruing the interest until that time. That is something on which you should get advice from your accountant.
So far as I know, Manager has no ability to automatically calculate accrued interest.