Hi @frifru!
cool! 
I have no direct answer to this problem, but perhaps you don’t need this dashboard link often? I think the goal is, to put your hard earned graphics on your sitemap: See “Showing diagrams in your openHAB sitemap” in https://community.openhab.org/t/influxdb-grafana-persistence-and-graphing from @ThomDietrich
In short: every diagramm on your grafana website offers a “share” option:
With this link (watch out: direct image rendering), it is possible to integrate the diagramm in the sitemap (Image refresh=60000 url=" … "). In this link itself, it is possible to configure the time range and viewing options.
Sadly, the rendering of the image is slow. I don’t know why, perhaps someone has a tip to increase the loading time of grafana charts.
Good day!