How to trigger a "read value"?

Yes, didn’t find the information in first place, but it is in the wiki:

knx="[<[(< autoRefresh>)]][< dptId>:]< mainGA>"

so knx="<(60)1.001:1/2/3" would read from GA1/2/3 every 60 Seconds with DPT 1.001 (I didn’t test this)