Hi DanielYuHong , if you did not clone PortalStart.mod to your own portal, that's still fine, but for this case, we don't need to create 2 separate projects as Customized Portal and another one is Business Project, just only one project is enough for your case - Business Project it depends on PortalTemplate, then put all your start processes to it.
Then please check and make sure the configuration of Standard process is configured to PortalTemplate
StandardProcess:
DefaultPages: ch.ivyteam.ivy.project.portal:portalTemplate
Then start your process by clicking on the icon "play" at end of the link of your start process link, you can quickly see that the link has a param as "embedInFrame"
Anyway, the process can start in the iframe, it must:
embedInFrame = true in the customfield of start process or task or case, check more details here: https://developer.axonivy.com/market-cache/portal/portal-guide/8.0.19/portal-developer-guide/iframe/index.html
In case you still cannot start it, could you please send me the example project which can reproduce your issue, I will try and let me know the detail of how it does not work for you.
Thank you so much for reporting!