Bing Webmaster Verification Code is unique to your site. If you did not verify your site, please check how to add a website to Bing Webmaster to verify your ownership.
If you already verified your ownership, please follow the steps below to have the verification code.
Step 1: Sign in Bing webmaster, or to create a new account by clicking "Sign up";
Step 2: In the Dashboard, click "Configure My Site" from the left navigation;
Step 3: Click "Verify Ownership" from the extended list;
Step 4: In Option 2, you will see the verification code.
Please note that the above method is only useful if you are trying to verify the ownership of your property on Bing (thanks for all the comments). If you have already verified, you may need to find the verification code from your website in the header section.
Method 1: If you use Yoast SEO Plugin, please click the "SEO" >> "General", and find the verification code in the Bing box.
Method 2: You could also find the verification code in the "Header Code" box. The "Header Code" box is located in different places depending your site's theme.
Method 3: Another possible place is the "Theme Header" file. Click "Appearance" >> "Theme Editor" >>"Theme Header" and check the code in the file.
If all the above methods do not work, the last option is to ask help from Bing supports for the verification codes. Login your Bing account, and click the "Help" on the top-right corner;
On the left menu, select "Getting Help & Support" >"Webmaster Support" > "Email Support", and fill out the contact form.
After you click the "Submit" button, you will get auto-message and probably you will get your verification code in 2 or 3 business days.