You can, and as Stan said the approach is to google it for the CSS approach, and the mirror that in Webflow’s designer.
But why would you want to put it inside?
It seems easier and more appropriate to wrap both the textbox and the button in a flex DIV, style it indended however you like, and make the textbox borderless.
You may still need to research some CSS techniques, but you save yourself the z-index challenge of having them overlay.