Thank you for your help!
I have a working solution now, that depends on some shell-scripts running from within the scripts folder. I get the key from the getAccessToken.sh:
#!/bin/bash
curl --location --request POST 'http://myxiboserverip/api/authorize/access_token' \
--header '' \
--form 'client_id= XXXX ' \
--form 'client_secret=XXXX' \
--form 'grant_type=client_credentials' > /openhab/conf/scripts/outfile
I then cut the token from the outfile with a JSONPATH-Transformation.
Might not be an elegant solution, but it works perfectly.
I use it to control a Digital Signage Display. I can now retire, unretire or push screens to my Digital Signage System via openHAB.