Go vs. return button in iOS keyboard for HTML input forms

There is a better and more "straight forward" way of changing the enter button text. Use the attribute enterkeyhint in the input tag like in the example shown below.

<input enterkeyhint="go">

You can find more documentation on the Mozilla site here.

Update 2020/2021

As Cameron Cobb pointed out, Safari Mobile supports the new HTML attribute enterkeyhint since version 13.7 ~ Sept. 2020 (https://caniuse.com/mdn-html_global_attributes_enterkeyhint).

The following values are possible (https://mixable.blog/ux-improvements-enterkeyhint-to-define-action-label-for-the-keyboard-of-mobile-devices/):

<input enterkeyhint="enter">
<input enterkeyhint="done">
<input enterkeyhint="go">
<input enterkeyhint="next">
<input enterkeyhint="previous">
<input enterkeyhint="search">
<input enterkeyhint="send">

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Original Answer

Aha...

The 'Go' button is only shown, if the <input> tag is inside a <form> tag (and the form tag has an action attribute). So, if you access your form elements afterwards with i.e. JavaScript, you can omit <form> tags.

'Go' button:

<form action="..." method="...">
   <input type="text"></input>
</form>

'return' button:

<input type="text"></input>

The important part is that the form tag has an action property. (action="#" is a noop.)

<form>
  <input type="text" />
</form>

<form action="#">
  <input type="text" />
</form>