I regret to share that Aspose.Slides does not provide facility to add placeholders in PPTX. Only auto shapes can be added in PPTX. However, Aspose.Slides does allow to access the existing placeholder inside presentation that are added using PowerPoint. Despite all this, Aspose.Slides still fulfill the requirement of finding shapes in slides for you and using them for your desired purpose. The Alternative Text property can be used in this regard. You can assign the user defined name to the shapes holding particular type of text frame whether they are in normal slides or in master slides. You can access the shapes via Alternative Text property and use them for your purpose. Please visit this documentation link for your kind reference. Please share, if I may help you further in this regard.