foot in the door, get one's - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Achieve an initial stage; succeed with a first step. For example, I think I could do well in an interview once I get my foot in the door with an appointment. This term alludes to the door-to-door salesperson or canvasser who blocks the door with one foot so it cannot be closed.