Microsoft Project API: Change of Custom Fields and Numbers

Hi Community,

We are trying to change Custom and Number Fields of Tasks through the Microsoft Project API. Therefore, we are checking out the Project. Then perform a merge Request on the following URL with the Following Body:

MERGE https://<tenant>.sharepoint.com/sites/<site>/_api/ProjectServer/Projects('xxxxxxx-xxxx-xxxx-xxxx-xxxxxxx')/Draft/Tasks('xxxxxxx-xxxx-xxxx-xxxx-xxxxxxx')

{

    "Custom_0f066cf14018ef118adc00155da88d13": true,

    "WorkMiliseconds": 100,

"Name": "test123"

}

The Name gets changed (so the API is working), the WorkMiliseconds and the Custom Field remain the same (0 and "false", we also tried like putting the vlaues for the number and the custom field in apostrophe). We also tried it for other Custom Fields with other types (e.g. Collection(edm.string)). Both Fields are changeable in the Project App.

Is there anything we need to consider when trying to change Custom Fields or Number Fields through the API?

Thanks in advance!

Version Project Professional 2013: 15.0.9999.1000