In MemHT 3.9 you can create you own fields.
Now when i create these and goto modify my profile with an empty field, for some reason the empty field automatically fills with my user name.

Do you know how i can stop this please?

link to site?

I will check it.

I test it and have no problem ^^
you created the field correctly?

Try this:
Title = Test
Name =test
Type = Input
Show in forum = No

Sounds like browser auto-complete to me...

Maybe you're using any name, which is already being used on the page.
You can take a print screen? (admin.php?page=users&op=profile)