I developed my first PowerApp custom control which will generate Auto Number based on the format that you specified. I find this always a missing functionality from MS to generate Auto Number based on the data in a record. I thought it would be useful tool for lot of customers. The following are some details about my control.

1. You need to specify the AutoNumber format in the following format
2. Use {Something} if you want include constant string in the Auto Number like Prefix or Suffix. For ex: {ACC}
3. Use [fieldname] if you want to include a field value in the Auto Number. For ex: [accountnumber]
4. Use [fieldname(fieldname)] if you want to include a field value from the look up field. For ex: [transactioncurrencyid(isocurrencycode)]
5. You can use any separator in between.
6. For ex if you provide in this format {ACC}-[accountnumber]-[parentaccountid(numberofemployees)] , the output would be ACC-1001-200 (based on the values of the particular record).

You can find the source code here

PCF Gallery link here