BloatLers to repair this you can replace the power button board and flex cable. [https://www.parts-people.com/blog/2013/08/28/dell-inspiron-15-3521-5521-power-button-removal-and-installation/|Here are the instructions] and the replacement part is available at places [https://www.ebay.com/itm/Dell-Inspiron-15-3521-Power-Button-Board-LS-9101P-W-Cable-F2-Y3-h17-/152269550449?_trksid=p2385738.m4383.l4275.c10|like this] and many more.
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Unless you have a board schematic which would show what pads are responsible for the power on state, you cannot turn it on without replacing the defective parts.
BloatLers to repair this you can replace the power button board and flex cable. [https://www.parts-people.com/blog/2013/08/28/dell-inspiron-15-3521-5521-power-button-removal-and-installation/|Here are the instructions] and the replacement part is available at places [https://www.ebay.com/itm/Dell-Inspiron-15-3521-Power-Button-Board-LS-9101P-W-Cable-F2-Y3-h17-/152269550449?_trksid=p2385738.m4383.l4275.c10|like this] and many more.