this is kind of broad question , depends on the type of field , Although there might be two possibilities.

1. add a custom field before or after some other field in existing widget.
2. create your own widget having a html attribute element and you can replace that with the data you want to show (in case you want) if you want to receive data you can just add an <input> </input> field of html

for 1st approach you inherit the qweb template of that widget in which you want to place your field there you can place your html element, for how to follow QWEB

something like :

<t t-extend="PaymentScreenWidget">

<t t-jquery="div.pos-payment-container" t-operation="prepend">

<input> </input

</t>

</t>

for 2nd search for how to create custom widget this might be helpful

Edit:

Some Widgets in POS:

• NumpadWidget

• PaypadWidget

• PaypadButtonWidget

• OrderWidget

• ActionBarWidget

• ProductCategoriesWidget

• PaymentScreenWidget

• 
  

you can see all widgets defined in POS in "point_of_sale/static/src/js/widgets.js"

to check their position in on screen "point_of_sale/static/src/xml/pos.xml"

for rendering and functionality of widgets check in

"point_of_sale/static/src/js/screens.js"