Tax Code in PO Invoice Tab

Hello Team,

I am creating PO, and manually entering tax code in Invoice Tab at Line Item. (Free text line without any Material Master)

But system doesn't accept this tax code, there is no error, but tax code disappears once I hit the 'Enter' key.

Could you help guide what could be the issue, and why system is not accepting Tax Code? How tax code is determined in PO?

TIA.

-Dhananjay