In this case, you’re perfectly fine using quotation marks, because what you’re essentially doing is quoting the text on the button.
In essence, there is little structural difference between the following three statements:
He said, “Don’t walk”.
The stop light says, “Don’t walk”.
The button is labelled “Don’t walk”.
The only minute difference is that in the third one, you are quoting text, rather than direct speech, so there should not be a comma before the quote (although I would guess, without knowing for sure, that at least some style guides would be fine with having one).
Since you are quoting a label, which is akin to a title, and using it not so much as quoted text as a ‘label-as-label’, however, you would be perfectly safe if you italicised it, too:
Click the red button located at the bottom left, labelled Staff Portal.
Which option you choose is a matter of preference; both are quite logical and if there is any advice in any style guides to avoid either, I am not aware of it.