user/pass fields restriction help

[oveis86]

Hi,
My reseller WHM doesn't accept usernames longer than 8 letters and the passwords must not contain the username. these restrictions are not in my WHMCS order form, so my clients choose any user name and passwords that cant be used in cPanel.
how can I put these restrictions on WHMCS order form? please advice asap.

Thank you in advance.

[John]

WHMCS generated the username automatically based on the domain and tld, so this isn't an issue.