You are going to need to write a custom phonegap plugin so that you can access custom methods you write in objective C.
The official phonegap documentation is here.
I'll briefly explain how you will do this.
In your javascript you will call this code :
PhoneGap.exec("OpenMailAppPlugin.openMailApp",parameter1);
In objective C you will create a new file OpenMailAppPlugin class. Read the link above for exact instuctions, but the important method will be soemthing like this.
-(void) openMailApp:(NSMutableArray*)paramArray withDict:(NSMutableDictionary*)options {
NSString *parameter1 = [paramArray objectAtIndex:0]; //recieves information from javascript function
NSURL* mailURL = [NSURL URLWithString: @"mailto:%@?cc=bar@example.com&subject=Greetings%20from%Cupertino!&body=Wish%20you%20were%20here!",paramter1];
[[UIApplication sharedApplication] openURL: mailURL];
}
additionally, you may be interested in sending information back to your phonegap application. You can do this by injecting a javascript call that sends parameters. In your objective C function you would do something like this.
NSString * jsCallBack = [NSString stringWithFormat:@"myJavascriptFunction('%@');",parameter];
[self.webView stringByEvaluatingJavaScriptFromString:jsCallBack];