I managed to do it!
I have an InnerBlocks tag that I want to pre-load with a template. This template will create a custom acf block with pre-populated acf fields.
This is how my InnerBlocks Template looks like:
InnerBlocks_template = [
['acf/custom-block', { 'data' : {'field-name': 'This is my pre-loaded value'}}]
]
