Specifically, the link to the Edit Default Value option is on the Configure Custom Field page under Default Configuration Scheme.
I have created a custom field to test wiki markup. I've gone into Configure Custom Field, then to the Edit Default Value, and in the Set Custom Field Defaults page I've added some sample info:
*This text should be bold.*
-This text should be deleted-
It appears on my Issue Screen correctly as:

When I go back into the Edit Default Value without making any changes, what appears as the input is:
<p><b>This text should be bold.</b><br/>
<del>This text should be deleted</del></p>
Without making changes, if I click "Set Default" to save this, it displays on my Issue Screen incorrectly as:

So if I need to adjust what is in the Edit Default Value, I end up recreating the input, instead of adding to it.
What am I doing wrong?
I am still new to this product, so if additional information is needed, please let me know.
Thank you.