Every transaction you enter in Manager is a double entry. You just do not have to make the debit and credit decisions. For example, when you enter a payment from a bank account for some expense, Manager credits the bank account and debits the relevant expense account. The reason you cannot make monetary transactions as journal entries is not because they are not double entries. It is so less experienced users do not make mistakes in determining what is the debit and what is the credit.
The first thing is to determine whether, under local law, the assets must be depreciated over time to recognize their cost. If so, there is nothing you can do. You will be governed by regulations, which generally do not allow a full year (or even a quarter or month) of depreciation if you only owned them for one day.
But, as described in the Guide mentioned previously, many tax authorities allow accelerated depreciation under certain conditions. You may be able to claim the full cost on the date of purchase. If the assets must be classified as fixed assets because of their lifetime and cost, but accelerated depreciation is allowed, simply enter the depreciation. It will show up in the automatic Fixed assets - depreciation expense account. Read this Guide: https://www.manager.io/guides/9119.
On the other hand, if local regulations permit, you might not have to treat the assets as fixed assets at all. If they fall below a certain threshold, you may be able to simple enter them as an ordinary purchase with a payment transaction.