You should never use a Suspense account. Suspense is an automatic account where Manager puts transactions it doesn’t know how to properly complete. Generally, this is a sign you have entered an unbalanced transaction, where debits do not equal credits. This way, information is preserved, and other accounts are not corrupted.
Once you have created a cash account under Settings > Chart of Accounts, you can open the Opening Balances Report in the Reports tab. Click on the blue balance and enter the amount.
However, please note that opening balances are only used when making the transition to Manager. The opening balance in any account, cash or other type, should be adjusted to the balance reflected in your previous accounting system. If you are starting a new business and making a deposit of cash for operating purposes, you would need to Receive Money in the account and allocate the contribution to owner’s equity or capital, depending on your legal structure.