Indicating mandatory field without labels

I have multiple forms in my application. I have used minimalistic view where there are no explicit labels for fields. For example, check wireframe displayed below: enter image description here

Here First Name, Last Name, and Username are mandatory fields.

However, due to "some" guidelines, I have to mention which fields are mandatory before the user would fill out the form.

What is the best way to indicate that these fields are mandatory?

I do not want to add labels to the text boxes just for this purpose, as I have used the same styling everywhere.

Here are two options that I tried:

enter image description here enter image description here

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