How to automatically enter the login user name in the text field?

Question

I wanted to create a form for registered and logged in users. How to automatically enter the login user name in the text field? What code and where to enter this field when creating the form? This field is not meant to change and should be retrieved from the Joomla user base.

Greetings

sylwekb · Accepted Answer

Thank you. The described method works. Is it possible to set a greyed out field that cannot be edited? Where should I place such a text box so that users cannot edit it?
e.g.
readonly="readonly"

Works ! Behaviors --> Custom tag atribute html --> add readonly