To set your own trust level, you need to create a security policy file and then specify this file in the security policy section of the application web.config.
For example, you create a file called web_custom.config, set there desired permissions and then refer to this file from within your web.config file:
<!-- Custom trust level for my application -->
<trustLevel name="Custom" policyFile="web_custom.config" />
<trust level="Custom" originUrl=""/>
For more information, please refer to this article:
ASP.NET Trust Levels and Policy Files