can we add custom field on checkout page using shopify post purchase UI Extension?

[ChetuDev]

can we add a custom field and perform ajax on the checkout page using Shopify post-purchase UI Extension?

[Zworthkey]

hii @ChetuDev 
Can you tell me which custom field you want to add to the checkout page? so,
I can help you.
Thank You.

[ChetuDev]

I have developed a public app, requirement is I want to add a custom third part loyalty gift card field on the checkout payment step after all taxes and shipping calculations.