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There are 2 methods to include an external Subdomain to Clickfunnels.
1) Straight through your Domain registrar.
2) Through Cloudflare (you can discover the walkthrough here: https://youtu.be/CkpMQh5TQoU )
This video is going to reveal you how to include an external subdomain to Clickfunnels through your domain registrar.
If you wish to link a custom-made domain rather, you can do that in likewise in 2 methods:
1) Straight through your Domain registrar (here is a tutorial on how you can do that: https://youtu.be/1wLu12c9Bic )
2) Through Cloudflare (you can follow this video to set it up: https://youtu.be/ik0VRSbF2Dw )