You want to add a custom URL to your form collection link.
To add a custom URL you need a Starter or Pro account. Here’s how: How do I change my Senja change plan?
Go to https://app.senja.io/
Upgrade to a paid plan
If you’re not already part of a paid Senja account you will need to upgrade.
Navigate to Custom Domain on the Senja menu
First, go to the Custom Domain option on the Senja menu. Like this:

Enter your subdomain
Type the subdomain you want to use into the field and click the Save button.
Add your DNS records
Add the DNS records we provide to your domain provider.
You can see where these details load below.
This image is an example, please use the details in your Senja account.
