If you want to offer WebMoney as a payment option to your Whmcs cart you need a gateway module that integrates with the merchant.webmoney.ru website. The integration is not quite obvious due the various purses that a merchant may have in the WebMoney account. You may take a look to the module that I build and that it working fine for Whmcs version 5.2.x.
Important! Please note that:
1. The plugin is based on http://www.whmcs.com/members/communityaddons.php?action=viewmod&id=227
2. This is only a sample and you are using it on your own risk.
3. The code is not written by a PHP programmer so it may not be optimal. Feel free to modify it as you like.
How to setup:
1. Modify the webmoney.php file and enter the API URL, API Username and API Password
2. Upload the files to the /your_whmcs_folder/modules/gateways
3. Go to Whmcs Admin -> Setup -> Payments -> Payment Gateways and activate Webmoney module
4. Fill in the required fields
5. When you are ready testing check Show on Order Form and Save Changes.
Download Webmoney Gateway Module.