Power BI API missing option to set CustomData in Gateway Datasource

We are using PBI Embedded with Service Principal auth. If we create a GW Datasource manually we get the option to set the "Connection string property" to "CustomData" rather than the default which is "EffectiveUserName". We need this for some reports that use CUSTOMDATA() in their DAX queries. I cannot find any way of setting this using the API - is it missing? We have code that successfully generates a new Gateway Datasource and permissions it with the SP but we cannot find any way to set the "Connection string property" to "CustomData" rather than the default which is "EffectiveUserName". In fact we cannot see the connection string at all through the API.