Site URL stored in database?

You can use an unbound field, here I added it to the SOOrder table:

using PX.Data;
using PX.Data.Update;
using PX.Objects.SO;

namespace PX.Objects
{
    // Acuminator disable once PX1016 ExtensionDoesNotDeclareIsActiveMethod extension should be constantly active
    public class SOOrderSiteURLExt : PXCacheExtension<SOOrder>
    {
        #region UsrSiteURL
        [PXString(255)]
        [PXUIField(DisplayName="Site URL")]
        [PXUnboundDefault(typeof(SiteURLConstant), PersistingCheck = PXPersistingCheck.Nothing)]
        public virtual string UsrSiteURL { get; set; }
        public abstract class usrSiteURL : PX.Data.BQL.BqlString.Field<usrSiteURL> { }
        #endregion

        public class SiteURLConstant : PX.Data.BQL.BqlString.Constant<SiteURLConstant>
        {
            public SiteURLConstant() : base(PXInstanceHelper.HostName) { }
        }
    }
}