Hi @Roshini 

UI Action Client checkbox should be true and onClick use openPopup() method.

I did similar requirement long back, Adding code below. Modify according to your requirement.

function openPopup() {
    var tableName = ''; //table name here
    var sysid = g_form.getUniqueValue();
    //Create and open the dialog form

    var dialog = new GlideDialogForm('Title', tableName); //Provide dialog title and table name
    dialog.setSysID(sysid); //Pass in sys_id to edit existing record, -1 to create new record
    dialog.addParm('sysparm_view', 'Reasons'); //Add view of the form
  
    dialog.setLoadCallback(function(iframeDoc) {
        // To get the iframe: document.defaultView in non-IE, document.parentWindow in IE
        var dialogFrame = 'defaultView' in iframeDoc ? iframeDoc.defaultView : iframeDoc.parentWindow;
        dialogFrame.g_form.setValue('u_number', g_form.getValue('u_number'));
		dialogFrame.g_form.setValue('u_state', 'Completed');
		dialogFrame.g_form.setMandatory('u_justification', true);
		dialogFrame.g_form.setReadOnly('u_select_user', true);
		dialogFrame.g_form.setReadOnly('u_hiring_manager', true);

        dialogFrame = null;
		
    });
    dialog.render(); //Open the dialog
	
	
}

Refer Below article 

https://servicenowguru.com/system-ui/glidedialogwindow-quickforms/