To add points to Topo GPS using an URL, you can use the following format:

https://www.topo-gps.com/add/point?lat=[latitude in degrees]&lon=[longitude in degrees]&title=[title]&description=[description]

To create an URL that imports custom geodata (gpx/kml/kmz) you first have to publish this data somewhere on the internet. Then you can create an URL of the following form:

https://www.topo-gps.com/import?url=[url]

Here [url] is the url encoded url of the geodata. If the user taps on this URL and Topo GPS is installed, the geodata published at [url] is automatically imported into Topo GPS.