Change manually Tax base amount in F-90

hello

I need to manually set the Tax base amount in trx code F-90 (Fixed Asset adquisition from purchase with vendor) but it seems to be impossible. I can set the tax amount manually but I cannot set manually the 'Tax base amount' that is relevant for legal pourposes in my country

does anybody knows how I can set in F-90 the field 'Tax base amount' ready to be manually populated?

thanks in advance

Rafa