[ByDesign] how to tune tax in new supplier Invoice without purchase(WebService, Odata, Or by-hand)

Dear Experts,

the title is my subject right way. When I've tried to write a new Invoice with no purchase order, I'm looking for how to tune tax from web service, Odata or by-hand.

1. Invoice Entry is Gross value and I put the gross price in items' record.

2. click the button 'Check', it's calculated Total Tax Amount automatically.

3. and that moment I want to tune the tax amount using External Service(Odata, WebService, or manually) if I could in some reasons. I attached one screenshot for more details.

I'm not sure if this question is right, I just want to get good tips for this.

Cheers.