we had requirement to implement CRM 2013 look alike modal window for some webresources. Following code can do the needful:
var DialogOption = new Xrm.DialogOptions;
DialogOption.width = 500; DialogOption.height = 420;
Xrm.Internal.openDialog(“Web Resource path”,
DialogOption,
null, null,
CallbackFunction);
function CallbackFunction(returnValue){ }
In our case, we needed custom UI like below:
Hope it helps!
MSDYNAMICSBLOG BY DEEPESH 
Hi, using the above approach, do you get any issue opening the webresource on an IFD CRM 2013. Works for OnPrem, but get a 500 error when IFD
Hi,
I have earlier implemented this in CRM 2013 IFD and seemed to work just fine.
Regards,
Deepesh
Thanks for the reply. Did the webresource you are calling have .html extension to it?
Yes, It was html webresource. Please drop me detailed scenario on my mail id: deepeshsomani@hotmail.com and i will see if I could help further.
is this supported method?
No Sagar, this is not supported using Internal namespace is not supported.
Hi deepesh,
Thanks for your post!
I am trying your way/path to open contact entity in modal window but unable to open and url is built correctly which if i open in new tab of browser that gets opened. Can you plz check this.
https://community.dynamics.com/crm/f/117/p/183714/457716#457716
This is unsupported approach and might have changed from CRM 2013 and up dependent on the update you are on or version of CRM.
I will suggest to use this:https://msdn.microsoft.com/en-us/library/jj602956.aspx#BKMK_OpenEntityForm
This is not modal to open a new form, but is supported way.
Hi deepesh, is there a way to pass custom parameters via url to the webresource which you are trying to open in the modal window?
Hi Vicky,
This is unsupported and i dont thing passing url works.
Regards,
Deepesh