we don’t have the option to create methods on table extension, but we can do that via class extension

How?

for example:: we need to view vendor group name for the vendor in vendtable form  


1-Create a new class and name it as <Classname>_<Extension>

so you can create this class with the below syntax 

public static class Vendtable_Extension
{
}
Note:: the prefix (_Extension) is a must 
2-Create the display methods in the class 
public static class Vendtable_Extension
{
[SysClientCacheDataMethodAttribute(true)]  //This attribute will cache your display method.
public static display Name VendGroupName(Vendtable _this)
{
return vendgrou::find(_this.vendgroup).name;
}
}
3-build project and check error if found 

4-Customize this method in your Form 
locate your form and create a new string to add this method in the form 

set the properties for the string as the below

Data method format must be ExtensionClassName::MethodName
Thanks.