fixed header not intialising on modal pop up

table fixed header plugin is not working on dialoge box(pop up window)

Could you give a little more info on this? Are you using a custom jquery header with a fixed table header (something like this) inside of it? Or are you using a modified version of the forge tool EasyFreezy?

Do you have the header you want implemented in the template for the pop up you are generating? Or are you trying to implement it in a generic pop up window?

Keep in mind the pop up behavior of OutSystems windows is more like an iFrame than a traditional pop up window. This has some benefits and drawbacks. It can make implementation that would be straightforward on a traditional page more tricky.