We are building an IT platform with custom software, the costs of which can be considered capex (capital expenditure) and capitalised on the balance sheet as an asset but I cannot see which account in QBO to do this?

It’s generally part of Fixed Assets Category Type.

Click Company Gear > select Chart of Account under Settings > New button > Category Type = Fixed Assets > Detail Type = Other fixed assets (or Machinery & Equipment) > Name = Capitalized Software

You will need to get advice directly from your Accountant as well as auditors if required.

Certain software development costs can be capitalized only when it meets certain specific thresholds.
1.Capitalization occurs once technological feasibility has been established. Before that, it’s just software development phase (like R&D) overhead expense cycle.
2.All capitalized costs are determined to be recoverable. There is a very small window of accumulated costs that can be capitalized. Nom is no more than two years.
3.Capitalization ends and amortization begins when the product is available for general release to customers.
4.Also refer to FAS86-amended