First download the workout you want by clicking on the respective link.
Click on the file after the downlod. If you use a file explorer you will probably find it in the folder downloads.
The Hiitmi Workout files (generated by the website as well as by the “share” function in the app) have a .hiitmi extension and normally the first time you open them you should be asked which app you want to use => in this case Hiitmi.
Of course it is possible that another setting is already set and another app is defined as default program. If this is the case.
Go to Settings => Apps => Other app (wrong app) => Under the heading App Settings you will find “Set as default” => Delete default values. These settings may vary depending on the smartphone manufacturer and system.