Sorry, I should have given instructions, my bad.
1. Create a new file in the whmcs root directory specifying namespinner as the template name - see the whmcs docs on creating pages, this doesnt really do anything, just displays the template.
2. Create a form with a textfield called searchstr, with the form action pointing to the new namespinner.php page you created.
3. Modify the namespinner.tpl page and put in your own opensrs reseller details.
Sequence -
Form posts searchstr value to namespinner.php (which doesnt do anything, only displays the template)
namespinner template file gets the posted search string and uses the opensrs domain_suggest api method to return the results.
If anything is unclear just let me know.