What is best practice to choose social profile username?

Dear all webmasters

I just want to know that what is the best username practice for SEO purpose having
.com at the end of the username or without having .com or .net whatever domain extension is.
for example
https://ift.tt/3fsibu9
https://ift.tt/35DgxBw

I am creating a social profile for my website https://ift.tt/35Dgy8y, therefore I am confused to choose a username

please suggest me a good way to choose the username related to the website which...

What is best practice to choose social profile username?

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