First, make sure that the custom field you've created has a check-mark. You can follow these steps in verifying it:
From the Gear icon, select the Custom form styles.
Locate the template and click Edit.
Go to the Content tab and and click the header section.
On the Display section, make sure that your custom field is checked. Click Done to save it.
Once done, let's see if we've selected the correct template when creating the estimate. Open the estimate you've created and click on Customize button at the bottom. Then, check if the check is in the right template.