Welcome to the Community. In QuickBooks, entering budget for fixed assets is unavailable. Though, you can add it under Description and name it.
Let me show you how to set up Fixed Assets:
Go to Setting, then choose Chart of Accounts.
Under Account Type drop-down arrow and select Fixed Asset.
From the Details Type drop-down arrow and choose that describes the asset.
Name the account, then under Description field you can name it Budget number.
Mark check the Track depreciation of this asset.
Enter the Original cost and the Depreciation.
Click Save and Close.
I understand the importance of this option when entering the budget number for you and your business. Rest assured, I'll personally send this feedback to our product developers. This way, it might be given a chance to be added to our next products update.
I've added an articles to learn more about fixed assets: