Form after submit

The Display text property can hold HTML markup so you can simply copy your message there as it is. If it is too long for that field, you can take advantage of resource strings, add a new resource string within the Localization application and then same localization macro in the Display text field.

But we can not paste into <span> some tags like <div> and so on. And if we look html that CMS generates we can see <div id="p_lt_ctl05_pageplaceholder_p_lt_ctl01_RegisterOnEventForm_viewBiz_pM_pMP"> <span id="p_lt_ctl05_pageplaceholder_p_lt_ctl01_RegisterOnEventForm_viewBiz_pM_lS" class="InfoLabel"> Thank you</span> </div>

Kentico processes them and add those IDs, but it shouldn't prevent you from styling them as you need. Alternatively, you can edit the web part itself and add your custom logic there.

The point is not about IDs, but in HTML semantics. If I paste <div> into "Display text" CMS generates

<span ID="bla bla bal" class="InfoLabel">
    <div>Thank you!</div>
</span>

And this construction contradicts to html standard.

Hi Dmitriy, another way for the post back is to create a thank you page which you can strip the page nesting, so it looks like a stand along page. You will have more control on that, even insert images as well. Then in the Form general property, set the post back to redirect to /thankyou page.

@Rui: I suppose Dmitriy needs the thank you message within the modal popup -> that's why I didn't suggest redirection to another page where he could format it.