Registration Custom Field

I would like to add a custom text field box to my registration page where it is madatory to provide a link to their sites.

I am using invision based theme.

Thanks

No as I clearly said that I want to add a custom field to my registration page where it is mandatory that they fill in their site.

the profile has nothing to do where it is mandatory for registration. Also where may I find to have it so I have to approve members.

Thank you

Deepest apologies sir.

I have tested your directions and all worked out!

Many thanks!