In order to allow your client to decide whether he wants to order server with or without license we can use Configurable Options (https://docs.whmcs.com/Addons_and_Configurable_Options).
Below we will show what steps to proceed to prepare such connection:
1.Configure CloudLinuxLicenses product as described here.
2.Go to Setup → Products/Services → Configurable Options and click Create a New Group.
3.Fill group name and add New Configurable Option, set up billing cycle, price and option type. Then save changes.
4.Go to Add-ons → CloudLinux Licenses Add-on → Configurable Options Relations and click Add Relation.
5.Choose appropriate configurable option and license product which it is assigned to and click Add relation.
Note. Plugin doesn’t support “quantity” type of Configurable Options.
Fig 6: Creating relation directly between server and license provisioning modules.