Can't hide form at the time it loads?

When a form is shown (you do call form.show, don't you?) the form is first loaded and after that it's shown which basically means that visible is set to true. So even if you hide (which actually sets the visible to false) the form in the load event the setting won't remain.

If you don't actually want to show the form yet, then I suggest that you don't call the Show method at this point. If you have some code you want to execute, create a new method and move the code into that method from the load event handler. After instantiating the form, call this new method. This way the form won't be shown at all until you later call Show at some point.

Is this the main form for the application? If so, you want to change its initial WindowState.[^]

If it's an additional form in an application, you're likely changing the form's state before it will have any impact. See Order of Events in Windows Forms.[^] and Windows Forms Events Lifecycle.[^]

In other words when the form's OnLoad method is called, it is already not visible, so calling Hide() or setting Visible=false may have no effect.

I think I know what you are saying. You may need a postback in order for the server to rewrite the html as hidden. You can do this by adding

AutoPostBack="true"

Some controls, like buttons, already do a postback so this is not needed.