In the capture_tools out-of-tree module, one block called tag_on_set_one_f and one block called file_source_on_tag can be used together to send a stream of IQ samples from a file whenever a GUI button is pushed.
Download this sample flow graph here: https://www.funwithelectronics.com/wordpress/other_data/burst_msg_source_c.grc
So tag_on_set_one_f will produce samples as fast a possible, and therefore a throttle block is needed after file_source_on_tag to keep the sampling rate under control.
file_source_on_tag produces zeros between “bursts”.
If you have another block which for instance produced a packet whenever receiving a stream tag, it can also be used together with tag_on_set_one_f so that when a button is pushed, a packet is transmitted.