From the documentation https://docs.mendix.com/appstore/modules/deep-link

Use string argument – If the selected microflow has a single string argument, the remainder of the invoking URL will be passed as an argument to the microflow. This way, it is possible to invoke microflows without having a corresponding object in the database. This property cannot be used in combination with the object type and object attribute properties.

or with the get parameters:

Include GET parameters – If you are using a string argument, the GET parameters following the URL will also be included in the string passed to the microflow. For example, with the http://appname/link/mfname/stringtext?param=value URL, previously only the string stringtext was passed to the microflow. When this option is enabled, the GET parameters will be added. The string passed to the microflow will now be stringtext?param=value. Of course, multiple GET parameters (using&) also work. This property cannot be used in combination with the object type and object attribute properties.
Separate GET parameters – If GET parameters are included, they are separated into multiple string parameters for the microflow that will be called. For example, with the http://appname/link/mfname/stringtext?param=value&other=test URL, the microflow that is called can receive two string parameters named param and other, which are filled with the values value and test, respectively. When creating a deep link in the GUI, an example URL will be shown for the selected microflow. This property cannot be used in combination with the object type and object attribute properties.

So just make sure the microflow that you are callin contains a string as input parameter or multiple string input parameters with the names defined when using the GET parameters option.