The autofill button is tied to the last field set in the lookup rule. Knowing that you can target all or just one of the autofill buttons and use the CSS below to change the text:

Add the custom-auto-fill class to the fields containing the autofill button.

.custom-auto-fill .auto-fill-btn-container > button {
    position: fixed;
    top: 12px;
    right: 19px;
  	visibility: hidden;
}
  
.custom-auto-fill .auto-fill-btn-container > button:after {
  	content: "Custom Text";
        visibility: visible;
        border: solid;
        padding: 6px 16px;
        background: var( --orange);
 	color: white;
}